usb: dwc2: gadget: add mutex to serialize init/deinit calls

This patch adds mutex, which protects initialization and
deinitialization procedures against suspend/resume methods. This mutex
will be needed by the updated suspend/resume calls, which tracks gadget
state.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>