mailbox: omap: Use function local struct mbox_controller

The mbox_controller struct is only needed in the probe function. Make
it a local variable instead of storing a copy in omap_mbox_device
to simplify that struct.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>