i2c: rtl9300: rename internal sda_pin to sda_num

i2c: rtl9300: rename internal sda_pin to sda_num

Rename the internally used 'sda_pin' to 'sda_num' to make it clear that
this is NOT the actual pin number of the GPIO pin but rather the logical
SDA channel number. Although the alternate function SDA_Y is sometimes
given with the GPIO number, this is not always the case. Thus, avoid any
confusion or misconfiguration by giving the variable the correct name.

This follows the description change in the devicetree bindings.

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/20250927101931.71575-4-jelonek.jonas@gmail.com