net: ksz9477: set i2c bus offset length only when needed

net: ksz9477: set i2c bus offset length only when needed

In order to add ksz9477 SPI bus support, check parent bus
is an I2C bus before calling i2c_set_offset_len().

Doing so, ksz_i2c_probe() will now return an error (-EINVAL) if
the parent bus is not the one expected by the ksz-switch u-boot
driver.

Indeed, the DSA KSZ devicetree binding doesn't specify anything
about the underlying bus between the SoC and the DSA switch, so
the same "compatible" string can be used wathever the management
interface used (SPI or I2C).

The ksz-switch u-boot driver currently only support I2C interface
but will match a compatible "microchip,ksz9xxx" located under
under an SPI bus node.

Signed-off-by: Romain Naour <romain.naour@smile.fr>