tbf: Allow multiple concurrent polls

There's no good reason to allow only for 1 concurrent POLL requested to
a TBF, it was onyl done this was as an implementation limitation factor.
It can well happen that several multiple POLLs may be in transit at the
same time, eg to get DL ACK/NACK as well as to get a CTRL ACK for a Pkt
Cell Change Continue (NACC).

Change-Id: Ic4080db684a4626cae90dd574d123081981284ca

src/tbf.cpp

@@ -557,10 +557,6 @@ int gprs_rlcmac_tbf::check_polling(uint32_t fn, uint8_t ts,
 			ts, control_ts);
 		return -EINVAL;
 	}
-	if (poll_state != GPRS_RLCMAC_POLL_NONE) {
-		LOGPTBF(this, LOGL_DEBUG, "Polling is already scheduled\n");
-		return -EBUSY;
-	}
 
 	if ((rc = pdch_ulc_get_next_free_rrbp_fn(trx->pdch[ts].ulc, fn, poll_fn_, rrbp_)) < 0) {
 		LOGPTBF(this, LOGL_DEBUG,