usb: dwc3-msm: Fix NULL pointer dereference in gsi_ring_db

If the obtained GSI door bell address in gsi_ring_db
is NULL, it will result in NULL pointer dereference.
Fix this by bailing out from the function if the
address is NULL.

Also change the dev_dbg to dev_err for more
visibility in case of error.

Change-Id: Idfb71cbdbd0b92d54f14951770e0961ab176ad6f
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>