[SCSI] scsi: fix lpfc build when wmb() is defined as mb()

On architectures where wmb() is defined as mb(), a build error
happens since there is also a variable named 'mb' in lpfc_sli.c's
lpfc_sli_issue_mbox_s3() function.  Rename the variable to 'mbx'
to prevent the build error.

drivers/scsi/lpfc/lpfc_sli.c: error: called object 'mb' is not a function

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: James Smart <james.smart@emulex.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>