usb: dwc2: gadget: force gadget initialization in dev mode

When booting with id pin grounded, dwc2 default to host mode.
Thus, force device mode prior initializing gadget part.
Else fifo init will fail since fifo values are not correct
in host mode.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>