l1sap: fix PTCCH detection
l1sap: fix PTCCH detection

The macro L1SAP_IS_PTCCH(fn) only detects a PTCCH channel at fn%52 = 12,
the detection logic has been extended in order to detect PTCCH at fn%52 = 38.

See also 3GPP TS 05.02, Clause 7, Table 6 of 9

Change-Id: Ia6f3333121e5e920c5e1d562a081d0a1b58a1153

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