net: pcs-xpcs: depend on MDIO_BUS instead of selecting it

The below compile time error can be seen when PHYLIB is configured as a
module.

 ld: drivers/net/pcs/pcs-xpcs.o: in function `xpcs_read':
 pcs-xpcs.c:(.text+0x29): undefined reference to `mdiobus_read'
 ld: drivers/net/pcs/pcs-xpcs.o: in function `xpcs_soft_reset.constprop.7':
 pcs-xpcs.c:(.text+0x80): undefined reference to `mdiobus_write'
 ld: drivers/net/pcs/pcs-xpcs.o: in function `xpcs_config_aneg':
 pcs-xpcs.c:(.text+0x318): undefined reference to `mdiobus_write'
 ld: pcs-xpcs.c:(.text+0x38e): undefined reference to `mdiobus_write'
 ld: pcs-xpcs.c:(.text+0x3eb): undefined reference to `mdiobus_write'
 ld: pcs-xpcs.c:(.text+0x437): undefined reference to `mdiobus_write'
 ld: drivers/net/pcs/pcs-xpcs.o:pcs-xpcs.c:(.text+0xb1e): more undefined references to `mdiobus_write' follow

PHYLIB being a module leads to MDIO_BUS being a module as well while the
XPCS is still built-in. What should happen in this configuration is that
PCS_XPCS should be forced to build as module. However, that select only
acts in the opposite way so we should turn it into a depends.

Fix this up by explicitly depending on MDIO_BUS.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Fixes: 2fa4e4b7 ("net: pcs: Move XPCS into new PCS subdirectory")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>