[SCSI] qla2xxx: Unify the read/write sfp mailbox command routines.

Make the read/write sfp mailbox command routines uniform, and remove redundancy.
Also protect against attempting to do a single byte dma in these routines.

Signed-off-by: Joe Carnuccio <joe.carnuccio@qlogic.com>
Signed-off-by: Madhuranath Iyengar <Madhu.Iyengar@qlogic.com>
Signed-off-by: James Bottomley <jbottomley@parallels.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>