sfc: Always close net device at the end of a disabling reset

This fixes a regression introduced by commit
eb9f6744 "sfc: Implement ethtool
reset operation".

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Cc: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>