gianfar: rx parser

Only let the rx parser be enabled if it is necessary (if VLAN extraction,
IP or TCP checksumming or the rx queue filer are enabled). Otherwise
disable it.

The new routine gfar_check_rx_parser_mode should be run after every
change on this features and will enable/disable the parser as necessary.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: Sebastian Poehn <sebastian.poehn@belden.com>
Signed-off-by: David S. Miller <davem@davemloft.net>