bus: mhi: host: Avoid ringing EV DB if there are no elements to process

Currently, mhi_process_data_event_ring()/mhi_process_ctrl_ev_ring() APIs
are ringing DB even if there are no ring elements to process. This could
cause the device to process the DB event in the absence of ring elements.
So to avoid this unnecessary device processing, let's ring event DB only
if there are any ring elements to process.

Signed-off-by: Vivek Pernamitta <quic_vpernami@quicinc.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/1680601458-9105-1-git-send-email-quic_vpernami@quicinc.com
[mani: massaged the commit message a bit]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>