sfc: fix dereference of table before it is null checked

Currently pointer table is being dereferenced on a null check of
table->must_restore_filters before it is being null checked, leading
to a potential null pointer dereference issue.  Fix this by null
checking table before dereferencing it when checking for a null
table->must_restore_filters.

Addresses-Coverity: ("Dereference before null check")
Fixes: e4fe938c ("sfc: move 'must restore' flags out of ef10-specific nic_data")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>