usb: gadget: f_hid: Use spinlock instead of mutex

commit 33e4c1a9987a1fc3b42c3b534100b5b006d55c61 upstream.

As IN request has to be allocated in set_alt() and released in
disable() we cannot use mutex to protect it as we cannot sleep
in those funcitons. Let's replace this mutex with a spinlock.

Tested-by: David Lechner <david@lechnology.com>
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>