PHYLINK takes care of filing the right information into
state->an_enabled, get rid of the read from the SerDes's BMCR register.

Fixes: 0e01491d ("net: dsa: b53: Add SerDes support")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>