i2c: rtl9300: split data_reg into read and write reg

i2c: rtl9300: split data_reg into read and write reg

In RTL9607C i2c controller, there are 2 separate registers for reads
and writes as opposed the combined 1 on rtl9300 and rtl9310.

In preparation for RTL9607C support, split it up into rd_reg and wd_reg
properties and change the i2c read and write functions accordingly.

Signed-off-by: Rustam Adilov <adilov@disroot.org>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20260401180648.337834-2-adilov@disroot.org