staging: gdm72xx: Remove unnecessary memset of netdev private data

The memory for struct net_device private data is allocated using
kzalloc/vzalloc in alloc_netdev_mqs, thus there is no need to zero it
again in the driver.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>