[SCSI] qla4xxx: Fix double clearing of risc_intr for ISP83XX

Issue:
For ISP83XX risc_intr getting clear two times in case of polling mode.
risc_intr getting clear from interrupt_service_routine() as well
from process_mbox_intr() because of this driver may clear risc_intr
without processing interrupt.

Fix:
Do not clear risc_intr from process_mbox_intr().

Signed-off-by: Manish Dusane <manish.dusane@qlogic.com>
Signed-off-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>