usb: gadget: fusb300_udc.h: Fix typo in include guard

Clearly this was meant to be an include guard, but a trailing
underscore was missing. It has been this way since the file was
introduced in 0fe6f1d1 ("usb: udc: add Faraday fusb300 driver").

Fixes: 0fe6f1d1

 ("usb: udc: add Faraday fusb300 driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Felipe Balbi <balbi@ti.com>