l1sap: Improve log msg when frame diff >1
l1sap: Improve log msg when frame diff >1

Print the two fn values to understand better the wrong behaviour.

Change-Id: I4f7b3ffbf7ce3a8d8d6872e4281ef228f4c5527f

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