net: pcs: xpcs: make the checks related to the PHY interface mode stateless

The operating mode of the driver is currently to populate its
struct mdio_xpcs_args::supported and struct mdio_xpcs_args::an_mode
statically in xpcs_probe(), based on the passed phy_interface_t,
and work with those.

However this is not the operation that phylink expects from a PCS
driver, because the port might be attached to an SFP cage that triggers
changes of the phy_interface_t dynamically as one SFP module is
unpluggged and another is plugged.

To migrate towards that model, the struct mdio_xpcs_args should not
cache anything related to the phy_interface_t, but just look up the
statically defined, const struct xpcs_compat structure corresponding to
the detected PCS OUI/model number.

So we delete the "supported" and "an_mode" members of struct
mdio_xpcs_args, and add the "id" structure there (since the ID is not
expected to change at runtime).

Since xpcs->supported is used deep in the code in _xpcs_config_aneg_c73(),
we need to modify some function headers to pass the xpcs_compat from all
callers. In turn, the xpcs_compat is always supplied externally to the
xpcs module:
- Most of the time by phylink
- In xpcs_probe() it is needed because xpcs_soft_reset() writes to
  MDIO_MMD_PCS or to MDIO_MMD_VEND2 depending on whether an_mode is clause
  37 or clause 73. In order to not introduce functional changes related
  to when the soft reset is issued, we continue to require the initial
  phy_interface_t argument to be passed to xpcs_probe() so we can pass
  this on to xpcs_soft_reset().
- stmmac_open() wants to know whether to call stmmac_init_phy() or not,
  and for that it looks inside xpcs->an_mode, because the clause 73
  (backplane) AN modes supposedly do not have a PHY. Because we moved
  an_mode outside of struct mdio_xpcs_args, this is now no longer
  directly possible, so we introduce a helper function xpcs_get_an_mode()
  which protects the data encapsulation of the xpcs module and requires
  a phy_interface_t to be passed as argument. This function can look up
  the appropriate compat based on the phy_interface_t.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>