[SCSI] qla2xxx: Refactor request/response-queue register handling.

Original code used an overabundance of indirect pointers to
function helpers.  Instead, the driver can exploit the immutable
properties of a queue's ISP-association and ID, which are both
known at queue initialization-time.

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