USB: gadgetfs: fix copy_to_user while holding spinlock

The gadgetfs driver as a long-outstanding FIXME, regarding a call of
copy_to_user() made while holding a spinlock.  This patch fixes the
issue by dropping the spinlock and using the dev->udc_usage mechanism
introduced by another recent patch to guard against status changes
while the lock isn't held.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
CC: <stable@vger.kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>