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.

Change-Id: Ic933da63cc2e7087b8e68398499d367fae328499
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Git-commit: 25cd9721c2b16ee0d775e36ec3af31f392003f80
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git


Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>