[SCSI] qla4xxx: Handle one H/W Interrupt at a time

Handle one H/W Interrupt at a time to prevent holding off H/W lock
for longer period of time.

Signed-off-by: Ravi Anand <ravi.anand@qlogic.com>
Signed-off-by: Vikas Chaudhary <vikas.chaudhary@qlogic.com>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>