phy: lynx-28g: improve phy_validate() procedure

lynx_28g_validate() suffers from the following shortcomings:

- Changing the protocol should not be possible if the source protocol of
  the lane is unsupported. This is because lynx_28g_proto_conf[] only
  covers the register deltas between any pair of supported lane modes,
  but that delta is probably incomplete if the source protocol is, say,
  PCIe (which is currently assimilated by the driver to
  LANE_MODE_UNKNOWN).

  lynx_28g_proto_conf() does refuse changing the protocol if the current
  one is unsupported, but we shouldn't advertise it via phy_validate()
  at all.

  The phy_set_mode_ext() call should perform the exact same
  verifications as phy_validate() did, in case the caller bypassed
  phy_validate(). So we need to centralize the logic into a common
  validation. But lynx_28g_set_mode() later needs the lane_mode that
  this validation needs to compute anyway, so name the common helper
  lynx_phy_mode_to_lane_mode() and let it return that lane_mode.

- Future core sanity checks on phy_validate() will want to differentiate
  the case where this optional method is not implemented from the case
  where the mode/submode is really not supported. So we shouldn't return
  -EOPNOTSUPP from lynx_28g_validate(), but -EINVAL to signal that we do
  implement the operation:
  https://lore.kernel.org/linux-phy/aY2lFTIALH7qEJmM@shell.armlinux.org.uk/

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20260610151952.2141019-14-vladimir.oltean@nxp.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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