sfc_ef100: implement ndo_open/close and EVQ probing

Channels are probed, but actual event handling is still stubbed out.

Stub implementation of check_caps is needed because ptp.c will call into
 it from efx_ptp_use_mac_tx_timestamps() to decide if it wants TXQs.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>