edge: Disable GPRS/EGPRS mixed mode

Currently the plain 'egprs' command enables EGPRS but doesn't prevent
phones from being served in GPRS mode if they do not support EGPRS.
This involves complex frame allocation implementations in dynamic
mode, especially if 8PSK is being used. This is due to the inability
of non-EGPRS phone to decode 8PSK USF and ES/P altogether. Since
polling has a higher priority than USF, collisions will have to be
prevented by the PCU by never using an GPRS USF if it refers to a FN
that is already being used for polling.

This commit just disables mixed usage by ignoring GPRS-only request
if EGPRS is enabled.

The following VTY command (config-pcu node) is changed:

  egprs     ->   egprs only

Sponsored-by: On-Waves ehf
diff --git a/tests/tbf/TbfTest.ok b/tests/tbf/TbfTest.ok
index 916ea52..c4bc870 100644
--- a/tests/tbf/TbfTest.ok
+++ b/tests/tbf/TbfTest.ok
@@ -26,3 +26,5 @@
 === end test_tbf_dl_flow_and_rach_single_phase ===
 === start test_tbf_dl_reuse ===
 === end test_tbf_dl_reuse ===
+=== start test_tbf_gprs_egprs ===
+=== end test_tbf_gprs_egprs ===