usb: gadget: s3c2410_udc: Remove static char buffer, use vsprintf extension %pV

Using unnecessary static char buffers isn't good.
Use the %pV extension instead.

Miscellanea:

o the dprintk return value is unused, make it void
o add __printf format and argument verification

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>