net: phy: sfp: make the i2c-bus dt property mandatory

This patch makes the i2c-bus property mandatory when using a device
tree. If the sfp i2c bus isn't described it's impossible to guess the
protocol to use for a given module, and the sfp module would then not
work in most cases.

Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>