usb: gadget: add multiple definition guards

If f_fs.c and u_serial.c are combined together using #include, which has
been a common practice so far, the pr_vdebug macro is defined multiple
times. Define it only once.

Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>