xen-netfront: Use static attribute groups for sysfs entries

Instead of manual calls of device_create_file() and
device_remove_files(), assign the static attribute groups to netdev
groups array.  This simplifies the code and avoids the possible
races.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>