net: sfp: use i2c_get_adapter_by_fwnode()

Use the newly introduced i2c_get_adapter_by_fwnode() API, so that we
can retrieve the I2C adapter in a firmware independent manner once we
have the fwnode handle for the adapter.

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>