Changes

library: add f_gsm_fn_{sum,sub,diff}()

Change-Id: Ie91ccef5d3f170f4a341a6cf9d3323a896b299e6

bts: TC_pcu_interf_ind: check reporting interval instead of absolute FN

Recent osmo-bts commit 53f0ec29 broke TC_pcu_interf_ind:

  Verdict: fail reason: Odd TDMA frame number := 935

There's no requirement that periodic interference reports must land
on a TDMA frame number satisfying 'fn mod 104 == 0' - only that they
keep arriving every configured averaging period.  Instead, verify
that the interval between consecutive reports equals 104 TDMA frames
(1 SACCH period), using the new f_gsm_fn_sub() helper.

Change-Id: Ia96f2926400bae8c0a6fb8f81eb56b48ca5f2de9
Related: 53f0ec29 ("l1sap: fix duplicate RF RESOURCE INDICATION on clock bootstrap")