ANDROID: usb: gadget: fix NULL pointer issue in mtp_read()

pointer dev->ep_out->desc is set to NULL if MTP function
is disabled during read operation. So we need to do pointer check
before access it and add spin lock protection in case it's modified
at another place in future.

Patchset: mtp

Change-Id: I96d3d685e93276c9065a1aa7b0cbbdc2e159aa6f
Signed-off-by: Jiebing Li <jiebing.li@intel.com>
Signed-off-by: Wang, Yu <yu.y.wang@intel.com>
Signed-off-by: Russ Weight <russell.h.weight@intel.com>