Remove needless use of mbox_t, replacing with just struct
mbox_out. Silences compiler warnings under a -Warray-bounds build:

drivers/scsi/megaraid.c: In function 'megaraid_probe_one':
drivers/scsi/megaraid.c:3615:30: error: array subscript 'mbox_t[0]' is partly outside array bounds of 'unsigned char[15]' [-Werror=array-bounds]
 3615 |         mbox->m_out.xferaddr = (u32)adapter->buf_dma_handle;
      |         ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/scsi/megaraid.c:3599:23: note: while referencing 'raw_mbox'
 3599 |         unsigned char raw_mbox[sizeof(struct mbox_out)];
      |                       ^~~~~~~~

Link: https://lore.kernel.org/r/20220105173633.2421129-1-keescook@chromium.org
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Sumit Saxena <sumit.saxena@broadcom.com>
Cc: Shivasharan S <shivasharan.srikanteshwara@broadcom.com>
Cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: megaraidlinux.pdl@broadcom.com
Cc: linux-scsi@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>