I2CBUS: Calculate I2C-settings as compiletime instead at runtime.

We can reduce the i2c-settings to just one byte, and this new const value
can directly be used by the I2C-peripheral without additional computation.
This saves 1-xdata load and a lot of bit-shifting.

I see a codesize savings of +200 bytes.

rtl837x_pins.h

@@ -71,12 +71,6 @@
 /* Not available GPIO */
 #define GPIO_NA                   0xFF
 
-/* Convert SDA PIN GPIO to I2C bus number */
-uint8_t i2c_bus_from_sda_pin(uint8_t sda_pin) __banked;
-
-/* Convert SCL PIN GPIO to I2C bus number */
-uint8_t i2c_bus_from_scl_pin(uint8_t scl_pin) __banked;
-
 /*
  * Setup a GPIO pin as input
  * pin: GPIO pin number 0-63