scsi: xen-scsifront: shost_priv() can never return NULL

There is no need to check whether shost_priv() returns a non-NULL value, as
the pointer returned is just an offset to the passed in parameter.

While at it replace an open coded shost_priv() instance.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230822064817.27257-1-jgross@suse.com


Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>