pcu_sock: Change log about tx PCH confirm INFO->DEBUG
pcu_sock: Change log about tx PCH confirm INFO->DEBUG

It's not something useful to see unless someone's really debugging that
part, and it shows up quite frequently.

Change-Id: I3c0dee36c7d34e6b1341b517ce3bcd1b275e69c1

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