usb: dwc3: msm: Add check before accessing GSI registers

Some targets doesn't support GSI feature.
When DWC3 drivers notification event received it's
directly accessing GSI register which is invalid for
platforms using BAM2BAM mode. Hence fix the issue by
having check to avoid accessing GSI registers.

Change-Id: I57ca5c964236479cde53e9384a19f30b2dc56270
Signed-off-by: Chetan C R <cchinnad@codeaurora.org>