For SR-IOV we will need to send events to event queues that belong to
VFs serviced by other drivers.  Change the parameters of
efx_generate_event() to allow this and declare it extern.

While we're at it, remove the existing declaration under the wrong
name efx_nic_generate_event().
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>