usb: gadget: f_mtp: move userspace interface to uapi

Move the most of linux/usb/f_mtp.h header to uapi.  Move the only
remaining structure definition into f_mtp.c, the only place that
uses it.

Change-Id: I952c1a9dc15c36bf295a0eb4d74b6b1ad912ed03
Signed-off-by: Colin Cross <ccross@android.com>