tbf: Add check_polling/set_polling

Currently the checks for and the actual polling is done in several
places by copy & paste of several lines of code. This hinders changes
of they polling is handled internally and also is likely source of
programming mistakes.

Separate this into a check_polling function, that checks whether
polling is possible and returns the FN and the RRBP to be used in
that case. Otherwise the cause is logged (LOGL_DEBUG) and a negative
error value is returned. There are no other side effect beside the
logging.

If the call is successful, the set_polling method can be used to
actually register the polling.

Extend the encoder functions' parameters lists by an rrbp parameter.

Sponsored-by: On-Waves ehf
8 files changed