net: mvneta: fix use of state->speed

When support for short preambles was added, it incorrectly keyed its
decision off state->speed instead of state->interface.  state->speed
is not guaranteed to be correct for in-band modes, which can lead to
short preambles being unexpectedly disabled.

Fix this by keying off the interface mode, which is the only way that
mvneta can operate at 2.5Gbps.

Fixes: da58a931 ("net: mvneta: Add support for 2500Mbps SGMII")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>