net: phylink: require supported_interfaces to be filled

We have been requiring the supported_interfaces bitmap to be filled in
by MAC drivers that have a mac_select_pcs() method. Now that all MAC
drivers fill in the supported_interfaces bitmap, it is time to enforce
this. We have already required supported_interfaces to be set in order
for optical SFPs to be configured in commit f81fa96d

 ("net: phylink:
use phy_interface_t bitmaps for optical modules").

Refuse phylink creation if supported_interfaces is empty, and remove
code to deal with cases where this mask is empty.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/E1q0K1u-006EIP-ET@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>