log reception of PCU_IF_MSG_PAG_REQ messages from osmo-pcu
log reception of PCU_IF_MSG_PAG_REQ messages from osmo-pcu

osmo-pcu currently sends paging requests in PCU_IF_MSG_DATA_REQ
messages, rather than PCU_IF_MSG_PAG_REQ. Clarify a comment which
already alluded to this, and leave an explicit log entry if a
PCU_IF_MSG_PAG_REQ message is received.

Change-Id: I75fd8f051f943d876b5614fa088fff7e56b310ab
Related: OS#3018

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