mailbox: Allow operation without .recv callback

mailbox: Allow operation without .recv callback

Some shared memory mailboxes may have empty receive operation,
because the data are polled by upper layers directly from the
shared memory region, and there is no completion interrupt or
bit of any sort. Allow empty .recv callback, and if the .recv
callback is empty, exit from mbox_recv() right away, because
any polling for completion here would be meaningless.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Alice Guo <alice.guo@nxp.com>