omap: mailbox: remove unnecessary arg for omap_mbox_msg_send

Also removed from tx_data

Signed-off-by: C A Subramaniam <subramaniam.ca@ti.com>
Acked-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>