i2c: mediatek: Add OFFSET_EXT_CONF setting back

In the commit be5ce0e9 ("i2c: mediatek: Add i2c ac-timing adjust
support"), we miss setting OFFSET_EXT_CONF register if
i2c->dev_comp->timing_adjust is false, now add it back.

Fixes: be5ce0e9 ("i2c: mediatek: Add i2c ac-timing adjust support")
Signed-off-by: Kewei Xu <kewei.xu@mediatek.com>
Reviewed-by: Qii Wang <qii.wang@mediatek.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>