atmel: fix netdev ops conversion

sparse says:

drivers/net/wireless/atmel.c:1501:3: warning: Initializer entry defined twice
drivers/net/wireless/atmel.c:1505:3:   also defined here

and it's correct; atmel has its own ndo_change_mtu and
shouldn't use eth_change_mtu.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>