[SCSI] megaraid: cleanup type issue in mega_build_cmd()

On 64 bit systems the current code sets 32 bits of "seg" and leaves the
other 32 uninitialized.  It doesn't matter since the variable is never
used.  But it's still messy and we should fix it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Adam Radford <aradford@gmail.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>