i2c: meson: fix clock setting overwrite

When the slave address is written in do_start(), SLAVE_ADDR is written
completely. This may overwrite some setting related to the clock rate
or signal filtering.

Fix this by writing only the bits related to slave address. To avoid
causing unexpected changed, explicitly disable filtering or high/low
clock mode which may have been left over by the bootloader.

Fixes: 30021e37 ("i2c: add support for Amlogic Meson I2C controller")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>