f_channel_needs_modify(): ask actual TCH lchan mode via VTY

I am introducing a BSC test case that runs through a assigning a TCH
channel for signalling, and then using Channel Mode Modify to change its
mode from signalling to speech. For this to work,
f_channel_needs_modify() needs to actually look up the channel mode of
the TCH and not assume that a TCH is always in speech mode.

Related: SYS#4895
Change-Id: If30e2cec65da91cb5899ee29e2afb6696437a4c9
1 file changed