coding: gsm0503_tch_{fr,hr}_encode(): add ability to emit BFI

Every BTS needs to have some graceful handling for the scenario
where it is time to send out a speech frame on TCH DL, but there is
no frame to be sent.  One possible solution is to transmit dummy
FACCH, but this option is unattractive for TCH/HS where FACCH
displaces two speech frames rather than one.  A more elegant
solution is to emit a speech frame with inverted CRC3, causing
the MS receiver to declare a BFI condition to its Rx DTX handler.
Setting all u(k) bits to 0 is one way to produce such an inverted-CRC
speech frame (normal TCH FR/HR CRC3 for an all-zeros frame would be
111), and this method is in fact what sysmoBTS PHY is observed to do.
Add the same ability to gsm0503_tch_{fr,hr}_encode() functions,
indicated by payload length of 0.

Change-Id: Iade3310e16b906efb6892d28f474a0d15204e861
---
M src/coding/gsm0503_coding.c
1 file changed, 43 insertions(+), 1 deletion(-)