mailbox: omap: Use mbox_bind_client

Use generic mbox_bind_client() to bind omap mailbox channel to a client.

mbox_bind_client is identical to the replaced lines, except that it:
 - Does the operation under con_mutex which prevents possible races in
   removal path
 - Sets TXDONE_BY_ACK if omap uses TXDONE_BY_POLL. omap uses
   TXDONE_BY_IRQ, so this check is not applicable.
 - Calls chan->mbox->ops->startup, if available. omap doesn't have, so
   this is not applicable.

Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>