mailbox: omap: Move fifo size check to point of use

The mbox_kfifo_size can be changed at runtime, the sanity
check on it's value should be done when it is used, not
only once at init time.

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