bssgp: Fix bssgp_tx_fc_bvc parameter type

Currently large values for Bmax default MS get sliced since a uint16_t is
used as the type of the corresponding parameter of bssgp_tx_fc_bvc.
GSM 48.018, 11.3.2 which in turn refers to 11.3.5 specifies a maximum
of 6MB (0xffff * 100).

This commit changes the type to uint32_t to cover the full value
range.

Sponsored-by: On-Waves ehf
diff --git a/tests/gb/gprs_bssgp_test.ok b/tests/gb/gprs_bssgp_test.ok
index 0392e6a..a011bee 100644
--- a/tests/gb/gprs_bssgp_test.ok
+++ b/tests/gb/gprs_bssgp_test.ok
@@ -7,5 +7,9 @@
 BSSGP primitive, SAP 16777221, prim = 11, op = 2, msg = 41 07 81 27 
 BSSGP primitive, SAP 16777221, prim = 11, op = 2, msg = 41 07 81 05 04 82 04 d2 
 ----- test_bssgp_status END
+----- test_bssgp_flow_control_bvc START
+Got message: 26 1e 81 2a 05 82 10 22 03 82 c0 40 01 82 08 11 1c 82 60 20 
+Got message: 26 1e 81 2a 05 82 10 22 03 82 c0 40 01 82 08 11 1c 82 60 20 3c 81 78 06 82 11 44 
+----- test_bssgp_flow_control_bvc END
 ===== BSSGP test END