ASoC: cygnus: Add EXPORT_SYMBOL for helper function

The helper function cygnus_ssp_set_custom_fsync_width() is intended
to be called from an ASoC machine driver, need to export symbol
if using modules.

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