soc: swr-mstr: Remove master access from slave api

The read/write api exposed by soundwire master is intended
to be used only for accessing soundwire slave registers.
Remove master access from the api, to avoid invalid access
due to slaves sending devnum as zero incorrectly.

Change-Id: Ia00179f87908f95f03ef027630d178016affdb18
Signed-off-by: Ramprasad Katkam <katkam@codeaurora.org>