usb: gadget: mtp: Move mutex_init() to alloc_inst_mtp_ptp()

Now mutex_init() is in function_alloc_mtp_ptp() which will be
called when USB composition change.

In the corner case, USB composition change when do mtp_read(),
the mutex lock is initialized when do mutex_lock().

Change-Id: I7a61f6fe6d8865462c5445f7075722f4f151b03f
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>