mailbox: omap: Reverse FIFO busy check logic

It is much more clear to check if the hardware FIFO is full and return
EBUSY if true. This allows us to also remove one level of indention
from the core of this function. It also makes the similarities between
omap_mbox_chan_send_noirq() and omap_mbox_chan_send() more obvious.

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