i2c: rtl9300: fix channel number bound check

Fix the current check for number of channels (child nodes in the device
tree). Before, this was:

if (device_get_child_node_count(dev) >= RTL9300_I2C_MUX_NCHAN)

RTL9300_I2C_MUX_NCHAN gives the maximum number of channels so checking
with '>=' isn't correct because it doesn't allow the last channel
number. Thus, fix it to:

if (device_get_child_node_count(dev) > RTL9300_I2C_MUX_NCHAN)

Issue occured on a TP-Link TL-ST1008F v2.0 device (8 SFP+ ports) and fix
is tested there.

Fixes: c366be72 ("i2c: Add driver for the RTL9300 I2C controller")
Cc: stable@vger.kernel.org # v6.13+
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/20250831100457.3114-2-jelonek.jonas@gmail.com