omap: mailbox: resolve hang issue

omap4 interrupt disable bits is different. On rx kfifo full, the mbox rx
interrupts wasn't getting disabled, and this is causing the rcm stress tests
to hang.

Signed-off-by: Hari Kanigeri <h-kanigeri2@ti.com>
Signed-off-by: Armando Uribe <x0095078@ti.com>
Signed-off-by: Fernando Guzman Lugo <fernando.lugo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>