i2c-gpio: Make some internal functions static

i2c_gpio_getsda() and i2c_gpio_getscl() are only used in this file.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Acked-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>