[SCSI] aacraid: rework packet support code

Received from Mark Salyzyn,

Replace all if/else packet formations with platform function calls. This is in
recognition of the proliferation of read and write packet types, and in the
need to migrate to up-and-coming packets for new products.

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