scsi: megaraid_mbox: Remove redundant initialization of pointer mbox

The pointer mbox is being initialized with a value that is never read and
it is being updated later with a new value.  The initialization is
redundant and can be removed.

Link: https://lore.kernel.org/r/20210420104919.376734-1-colin.king@canonical.com
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Addresses-Coverity: ("Unused value")