net: dsa: bcm_sf2_cfp: fix an incorrect NULL check on list iterator

The bug is here:
	return rule;

The list iterator value 'rule' will *always* be set and non-NULL
by list_for_each_entry(), so it is incorrect to assume that the
iterator value will be NULL if the list is empty or no element
is found.

To fix the bug, return 'rule' when found, otherwise return NULL.

Fixes: ae7a5aff

 ("net: dsa: bcm_sf2: Keep copy of inserted rules")
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Link: https://lore.kernel.org/r/20220328032431.22538-1-xiam0nd.tong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>