pcu_sock: use llist_for_each_entry() in pcu_sock_close()
pcu_sock: use llist_for_each_entry() in pcu_sock_close()

Using gsm_bts_trx_num() involves redundant iterations over the
list of transceivers - we definitely don't want them.

Change-Id: I4bd40ffcc1e925412a21b0a934bbfdeddbc6ad1f

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