net: mvneta: Fix 10Mbps speed

In mvneta_adjust_link() we need to set MII_SPEED bit only if PHY reports
the speed at 100Mbps.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>