bus: mhi: Add mhi_queue_is_full function

This function can be used by client driver to determine whether it's
possible to queue new elements in a channel ring.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/1604424234-24446-1-git-send-email-loic.poulain@linaro.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>