net: pcs: xpcs: depends on PHYLINK in Kconfig

This is yet another attempt at fixing:

>> ERROR: modpost: "phylink_mii_c22_pcs_encode_advertisement" [drivers/net/pcs/pcs_xpcs.ko] undefined!
>> ERROR: modpost: "phylink_mii_c22_pcs_decode_state" [drivers/net/pcs/pcs_xpcs.ko] undefined!

Switch XPCS to be invisible, as Russell points out it's
"selected" by its consumers. Drop the dependency on MDIO_BUS
as "depends" is meaningless on "selected" symbols.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: b47aec88

 ("net: pcs: xpcs: add CL37 1000BASE-X AN support")
Link: https://lore.kernel.org/netdev/20220620201915.1195280-1-kuba@kernel.org/
Link: https://lore.kernel.org/r/20220622083521.0de3ea5c@kernel.org/
Link: https://lore.kernel.org/r/20220623202933.2341938-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>