l1sap: fix TDMA frame number arithmetic in fn_ms_adj()
l1sap: fix TDMA frame number arithmetic in fn_ms_adj()

Using the normal arithmetic for TDMA frame numbers is wrong.

Change-Id: Ie17670d150489014a4a2e140cede42086100bbc0

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