osmo-bts-trx: fix spurious clock skew shutdown after self-compensation
When the BTS runs ahead of the transceiver (elapsed_fn < 0), trx_sched_clock() reschedules the timerfd to deliberately delay the next FN. osmo_timerfd_schedule() resets the timerfd and discards any accumulated expirations, but last_fn_timer.tv was left pointing at the previous callback. The next trx_fn_timer_cb() then measures elapsed_us all the way back to that previous callback - spanning the deliberate delay (or any OS stall that preceded us) - and falsely trips the "PC clock skew too high" check, shutting the BTS down for no good reason.
Advance last_fn_timer.tv to the projected firing time of the rescheduled timer so that the next callback measures roughly one FN interval, as expected.
