assignment_fsm: Properly support assigning signalling mode TCH/x

To support the 3 possible preferences, the changes needed were:
 - Replace 'full_rate' bool with a 3 option enum to represent
   the channels types for signalling
 - Switch from _pref/_alt to using an array sorted in preference
   order

Change-Id: I4c7499c8c866ea3ff7b1327edb3615d003d927d3
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
8 files changed