[SCSI] aacraid: merge rx and rkt code

Received from Mark Salyzyn:

The only real difference between the rkt and rx platform modules is the
offset of the message registers. This patch recognizes this similarity
and simplifies the driver to reduce it's code footprint and to improve
maintainability by reducing the code duplication.

Visibly, the 'rkt.c' portion of this patch looks more complicated than
it really is. View it as retaining the rkt-only specifics of the
interface.

Signed-off-by: Mark Haverkamp <markh@osdl.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>