usb: gadgetfs: Convert semaphore to mutex

The semaphore data->lock is semantically a mutex. Convert it to a real
mutex.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>