common: track whether gsm_time has been initialized
l1sap_info_time_ind() used 'bts->gsm_time.fn != 0' as a proxy for "we have a previous frame number to diff against". This is unreliable: Fn=0 is a _valid_ frame number, recurring on every hyperframe wrap. If gsm_time.fn happened to be 0 and the next time indication jumped forward by more than one frame, the real gap was silently swallowed.
It also gave no clean way to suppress the bogus "Invalid condition detected: Frame difference is ..." message that appears when the PHY (re)starts its TDMA frame number (e.g. from 0) on bring-up.
Introduce an explicit 'bts->gsm_time_valid' flag instead:
* l1sap_info_time_ind() treats the first indication of an epoch as having no gap (frames_expired = 0): no warning, no RACH-slot accounting; * the flag is cleared in st_op_disabled_notinstalled_on_enter(), so each BTS bring-up starts a fresh clock epoch regardless of which FN the PHY reports first.
