sfc: don't double-down() filters in ef100_reset()

dev_close(), by way of ef100_net_stop(), already brings down the filter
 table, so there's no need to do it again (which just causes lots of
 WARN_ONs).
Similarly, don't bring it up ourselves, as dev_open() -> ef100_net_open()
 will do it, and will fail if it's already been brought up.

Fixes: a9dc3d56 ("sfc_ef100: RX filter table management and related gubbins")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>