usb: dwc3-msm: Avoid access of gsi_reg for non-GSI targets

Some targets do not have GSI feature. For such targets, gsi_reg
array does not get allocated. DWC3_CONTROLLER_NOTIFY_CLEAR_DB
event from gadget driver will access this unallocated memory.
Fix this by putting a check for gsi_reg.

Change-Id: I9903c0c6533abf3d6e689a40ea84ecbc066e10d8
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>