i2c: mediatek: use of_device_get_match_data()

The usage of of_device_get_match_data() reduce the code size a bit.

Also, the only way to call mtk_i2c_probe() is to match an entry in
mtk_i2c_of_match[], so of_id cannot be NULL.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>