msm: mhi_dev: Adding check for event ring

During the initialization, device waits for M0 state. Ring structures are
not yet allocated at this point. While waiting if device receives a D3cold
or D3hot event, device enters the sys err state. At the end of sys err
handling it acceses the event ring in mhi_dev_send_multiple_tr_events API
which is causing device crash.

Adding check for event ring in send multiple tr events API to fail
it if ring is not present.

Change-Id: Iffe44107c1925e0cccb38651aa09ba1f893f1014
Signed-off-by: Sai Chaitanya Kaveti <quic_skaveti@quicinc.com>