usb: dwc3: Request dwc3 irq in driver probe

request_irq() for dwc irq is called in dwc3_gadget_start()
when msm-dwc3 is in suspended state because usb cable is not
connected. dwc3 resume happens upon USB connect, enables
already enabled irq. This result into unbalanced enable irq
warning. Fix this by moving request_irq to dwc3_probe().

CRs-Fixed: 999124
Change-Id: I61ec085778fe43b87de18a84829ea3f5280c451c
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>