ASoC: cygnus: Remove set_fmt from SPDIF dai ops

The SPDIF port cannot modify its format so a set_fmt function is not
needed. Previously, we used a generic set_fmt for all ports and returned
an error code for the SPDIF port. It is cleaner to not populate the
set_fmt field.

Signed-off-by: Lori Hikichi <lori.hikichi@broadcom.com>
Signed-off-by: Mark Brown <broonie@kernel.org>