i2c: rtl9300: move setting SCL frequency to config_io

i2c: rtl9300: move setting SCL frequency to config_io

Move the register operation to set the SCL frequency to the
rtl9300_i2c_config_io function instead of the rtl9300_i2c_config_xfer
function. This rather belongs there next to selecting the current SDA
output line.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Tested-by: Sven Eckelmann <sven@narfation.org>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz> # On RTL9302C based board
Tested-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20250927101931.71575-5-jelonek.jonas@gmail.com