usb: gadget: f_hid: fix: Don't access hidg->req without spinlock held

hidg->req should be accessed only with write_spinlock held as it is
set to NULL when we get disabled by host.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>