Change the Kconfig dependency so that it doesn't require both I2C and SPI
subsystems to be built. Make a few small changes to the code so that the
code for a bus is only called if the bus is being built.

When SPI support was added to serial-multi-instantiate it created a
dependency that both CONFIG_I2C and CONFIG_SPI must be enabled.
Typically they are, but there's no reason why this should be a
requirement. A specific kernel build could have only I2C devices
or only SPI devices. It should be possible to use serial-multi-instantiate
if only I2C or only SPI is enabled.

The dependency formula used is:

  depends on (I2C && !SPI) || (!I2C && SPI) || (I2C && SPI)

The advantage of this approach is that if I2C=m or SPI=m then
SERIAL_MULTI_INSTANTIATE is limited to n/m.
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20240814132939.308696-1-rf@opensource.cirrus.comReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>