omap mailbox uses rq->data as the second opaque pointer to carry
mbox_msg_t and rq->special message argument which is needed only for
tx.  Add and use omap_msg_tx_data struct for tx and use rq->special
for mbox_msg_t for rx such that only rq->special is used as opaque
pointer.

[ Impact: cleanup rq->data usage, extra kmalloc in msg_send ]
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Russell King <rmk@arm.linux.org.uk>