sfc: use MCDI epoch flag to improve MC reboot detection in the driver

The Huntington MC will reject all MCDI requests after an MC reboot until it sees
one with the NOT_EPOCH flag clear.  This flag is set by default for all requests,
and then cleared on the first request after we detect that an MC reboot has
occurred.

The old MCDI_STATUS_DELAY_COUNT gave a timeout of 10ms, which was not long enough
for the driver to detect that a reboot had occurred based on the warm boot count
while calling efx_mcdi_poll_reboot() from the loop in efx_mcdi_ev_death().

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>