ASoC: wsa883x: add function to get swr device number

Add a function in wsa883x which returns the sound wire
device number for a given wsa883x codec slave instance.

Change-Id: Ie3d0e309870f488b1643a853bcddcf93314d94b5
Signed-off-by: Vignesh Kulothungan <vigneshk@codeaurora.org>