ul_tbf: Clean up handle_tbf_reject()

Document the function, make it look similar to usual TBF creation path
tbf_alloc_ul()->tbf_alloc_ul_tbf->tbf::setup(), which it mimics with
some differences.
Get rid of unneeded stuff like creating MS and settings its TLLI (that's
already done in only caller of the function). There's no need for
calling update_ms() either.

Change-Id: I61df2e4f0f0df1f8db941741a2d35a2319252c5e
diff --git a/tests/tbf/TbfTest.err b/tests/tbf/TbfTest.err
index 4b38aec..e700fe2 100644
--- a/tests/tbf/TbfTest.err
+++ b/tests/tbf/TbfTest.err
@@ -7956,8 +7956,8 @@
 [UL] algo A <multi> (suggested TRX: 0): failed to allocate a TS, no USF available
 TBF(TFI=0 TLLI=0xffeeddd3 DIR=UL STATE=NULL EGPRS) Timeslot Allocation failed: trx = 0, single_slot = 0
 MS(TLLI=0xffeeddd3, IMSI=, TA=7, 11/11,) No PDCH resource
+MS(TLLI=0xffeeddd3, IMSI=, TA=7, 11/11,) Attaching UL TBF: TBF(TFI=0 TLLI=0xffeeddd3 DIR=UL STATE=NULL)
 TBF(TFI=0 TLLI=0xffeeddd3 DIR=UL STATE=NULL) changes state from NULL to ASSIGN
-MS(TLLI=0xffeeddd3, IMSI=, TA=7, 11/11,) Attaching UL TBF: TBF(TFI=0 TLLI=0xffeeddd3 DIR=UL STATE=ASSIGN)
 TBF(TFI=0 TLLI=0xffeeddd3 DIR=UL STATE=ASSIGN) changes UL ASS state from GPRS_RLCMAC_UL_ASS_NONE to GPRS_RLCMAC_UL_ASS_SEND_ASS_REJ
 PDCH(bts=0,trx=0,ts=7) Expiring FN=82 but previous FN=2654231 is still reserved!
 PDCH(bts=0,trx=0,ts=7) Timeout for registered POLL (FN=2654231): TBF(TFI=6 TLLI=0xffeeddd2 DIR=UL STATE=ASSIGN EGPRS)
@@ -7974,8 +7974,8 @@
 === start test_packet_access_rej_prr_no_other_tbfs ===
 Creating MS object, TLLI = 0xffffffff
 Modifying MS object, UL TLLI: 0xffffffff -> 0xffeeddcc, not yet confirmed
+MS(TLLI=0xffeeddcc, IMSI=, TA=220, 0/0,) Attaching UL TBF: TBF(TFI=0 TLLI=0xffeeddcc DIR=UL STATE=NULL)
 TBF(TFI=0 TLLI=0xffeeddcc DIR=UL STATE=NULL) changes state from NULL to ASSIGN
-MS(TLLI=0xffeeddcc, IMSI=, TA=220, 0/0,) Attaching UL TBF: TBF(TFI=0 TLLI=0xffeeddcc DIR=UL STATE=ASSIGN)
 TBF(TFI=0 TLLI=0xffeeddcc DIR=UL STATE=ASSIGN) changes UL ASS state from GPRS_RLCMAC_UL_ASS_NONE to GPRS_RLCMAC_UL_ASS_SEND_ASS_REJ
 TBF(TFI=0 TLLI=0xffeeddcc DIR=UL STATE=ASSIGN) starting timer T0 [reject (PACCH)] with 0 sec. 2000 microsec, cur_fn=2654167
 PDCH(bts=0,trx=0,ts=7) FN=2654218 Scheduling control message at RTS for TBF(TFI=0 TLLI=0xffeeddcc DIR=UL STATE=ASSIGN)