drivers: ssd1306: cleanup and make functions static
drivers: ssd1306: cleanup and make functions static

Cleanup and make functions static.

Signed-off-by: Johann Fischer <b39b26e1812eb092eb217277792f3735321fa456@phytec.de>

finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr