usb: gadget: s3c2410_udc: Replace <asm/io.h> with <linux/io.h>

Fixes the following warning:
WARNING: Use #include <linux/io.h> instead of <asm/io.h>

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>