usb: dwc3: gadget: add missing spin_lock()

commit 8e74475b (usb: dwc3: gadget: use udc-core's
reset notifier) added support for the new UDC core's
reset notifier to dwc3 but while at it, it removed
a spin_lock() from dwc3_reset_gadget() which might
cause an unbalanced spin_unlock() further down the line

Fixes: 8e74475b (usb: dwc3: gadget: use udc-core's reset notifier)
Cc: <stable@vger.kernel.org> # v3.19
Signed-off-by: Felipe Balbi <balbi@ti.com>