soundwire: Fix a signedness bug

"ret" is an int and "buf" is a u8.  sdw_read() returns negative error
codes which are truncated to the u8, 0-255 range before being stored as
an int.  It means that "ret" can't be less than zero.

Fixes: b0a9c37b ("soundwire: Add slave status handling")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>