sfc: Rename struct efx_mcdi_phy_cfg to efx_mcdi_phy_data

Most of its members are constant capabilities, not configuration.  The
new name is also consistent with the name of the pointer to it in
struct efx_nic and the names of structures used by other PHY drivers.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>