sgsn: Fix T3395 firing forever

field pdp->num_T_exp was being reset to 0 every time
pdpctx_timer_start() was called from gsm48_tx_gsm_deact_pdp_req().

Take the chance to test max amount of retrans to 4 as detailed in specs.

Change-Id: Iacce3c66f61578ebee37abaa287f7e183f985c1c
1 file changed