net: sfp: create/destroy I2C mdiobus before PHY probe/after PHY release

Instead of configuring the I2C mdiobus when SFP driver is probed,
create/destroy the mdiobus before the PHY is probed for/after it is
released.

This way we can tell the mdio-i2c code which protocol to use for each
SFP transceiver.

Move the code that determines MDIO I2C protocol from
sfp_sm_probe_for_phy() to sfp_sm_mod_probe(), where most of the SFP ID
parsing is done. Don't allocate I2C bus if no PHY is expected.

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>