OMAP: mailbox: change full flag per mailbox queue instead of global

The variable rq_full flag is a global variable, so if there are multiple
mailbox users there will be conflicts. Now there is a full flag per
mailbox queue.

Reported-by: Ohad Ben-Cohen <ohad@wizery.com>
Signed-off-by: Fernando Guzman Lugo <x0095840@ti.com>
Signed-off-by: Hari Kanigeri <h-kanigeri2@ti.com>
Acked-by: Hiroshi Doyu <hiroshi.doyu@nokia.com>