usb gadget: fix langwell_udc.c build error

Move function to fix langwell_udc.c build error:

drivers/usb/gadget/langwell_udc.c: In function 'show_langwell_udc':
drivers/usb/gadget/langwell_udc.c:1693:3: error: implicit declaration of function 'lpm_device_speed'
drivers/usb/gadget/langwell_udc.c: At top level:
drivers/usb/gadget/langwell_udc.c:2637:37: error: conflicting types for 'lpm_device_speed'
drivers/usb/gadget/langwell_udc.c:1693:20: note: previous implicit declaration of 'lpm_device_speed' was here

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Felipe Balbi <balbi@ti.com>
Cc:	linux-usb@vger.kernel.org