[SCSI] qla2xxx: Don't explicitly read mbx registers while processing a system-error.

Callers of qla2x00_async_event() already populate the mb[] array
upon invocation, doing so via the appropriate mailbox register
accessors.  The stale codes removed are leftover-bits kept during
the FWI2 transition.  Though relatively benign, the extra-reads
are not valid for FWI2 boards (ISP24xx and above) and peek into
the incorrect regions of registers.

Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>