tbf: Add frames_since_last_poll method

This functions calculates the number of frames that have passed since
the last DL poll (RRBP flag set) has been sent. It returns a value
less than zero (fn_now - fn_sched) if the block has been scheduled but
not yet sent.

If the function is called before the first data block has been sent
it will return -1.

If the function is called before the first DL poll is sent, it
returns the number of frames since the first data block has been
sent.

Sponsored-by: On-Waves ehf
3 files changed