net: stmmac: convert half-duplex support to positive logic

Rather than detecting when half-duplex is not supported, and clearing
the MAC capabilities, reverse the if() condition and use it to set the
capabilities instead.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qZAXn-005pUb-SP@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>