usb: mtu3: fix missing include of mtu3_dr.h

The declarations of ssusb_gadget_{init,exit} are
in the mtu3_dr.h file but the code does that implements
them does not include this. Add the include to fix the
following sparse warnigns:

drivers/usb/mtu3/mtu3_core.c:825:5: warning: symbol 'ssusb_gadget_init' was not declared. Should it be static?
drivers/usb/mtu3/mtu3_core.c:925:6: warning: symbol 'ssusb_gadget_exit' was not declared. Should it be static?

Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>