realtek: i2c: fix i2c-shared-gpio range check

realtek: i2c: fix i2c-shared-gpio range check

The i2c-shared-gpio driver is designed to emulate up to four
i2c busses with distinct sda lines and a a shared scl line.
For some reason the check for the number of allowed busses
is one off and the driver can only allocate three busses.
Fix that.

Fixes: acd7ecc9ed8 ("realtek: add new i2c-gpio-shared driver")
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/22543
Signed-off-by: Robert Marko <robimarko@gmail.com>