net: split netdev features to separate header

Move features definitions to separate header so that linux/skbuff.h won't
need to include linux/netdevice.h after netdev_features_t is introduced.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>