net: marvell: prestera: fix incorrect return of port_find

In case, when some ports is in list and we don't find requested - we
return last iterator state and not return NULL as expected.

Fixes: 501ef306

 ("net: marvell: prestera: Add driver for Prestera family ASIC devices")
Signed-off-by: Yevhen Orlov <yevhen.orlov@plvision.eu>
Link: https://lore.kernel.org/r/20211216170736.8851-1-yevhen.orlov@plvision.eu


Signed-off-by: Jakub Kicinski <kuba@kernel.org>