sfc: check warm_boot_count after other functions have been reset

A change in MCFW behaviour means that the net driver must update its record
of the warm_boot_count by reading it from the ER_DZ_BIU_MC_SFT_STATUS
register.

On v4.6.x MCFW the global boot count was incremented when some functions
needed to be reset to enable multicast chaining, so all functions saw the
same value.  In that case, the driver needed to increment its
warm_boot_count when other functions were reset, to avoid noticing it later
and then trying to reset itself to recover unnecessarily.

With v4.7+ MCFW, the boot count in firmware doesn't change as that is
unnecessary since the PFs that have been reset will each receive an MC
reboot notification.  In that case, the driver re-reads the unchanged
value.

Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>