soundwire: remove an unnecessary NULL check

The "bus" pointer isn't NULL so the address to a non-zero offset in
middle of "bus" cannot be NULL.  Delete the NULL check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200923083235.GB1454948@mwanda


Signed-off-by: Vinod Koul <vkoul@kernel.org>