net: mvneta: Fix clearing of MIB statistics

The existing function to clear the MIB statatistics was using the
wrong address for the registers. Also, the counters would of been
cleared when the interface was brought up, not during the
probe. Fix both of these.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>