l1sap: Don't enqueue PTCCH blocks for loopback
l1sap: Don't enqueue PTCCH blocks for loopback

When we're in loopback testing mode for PDTCH, we must make sure to
avoid adding PTCCH blocks to the queue. Only PDTCH blocks must be
enqueued. For the transmit (downlink) side, we already had the PTCCH
check in place.

Change-Id: I7ef40d9bdf74a99375bc6568ed9483499664bf6f

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts