net: dsa: b53: clean up if() condition to be more readable

I've stared at this if() statement for a while trying to work out if
it really does correspond with the comment above, and it does seem to.
However, let's make it more readable and phrase it in the same way as
the comment.

Also add a FIXME into the comment - we appear to deny Gigabit modes for
802.3z interface modes, but 802.3z interface modes only operate at
gigabit and above.

Reviewed-by: Florian Fainelli &lt;f.fainelli@gmail.com&gt; Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>