devlink: fix typo in documentation

This commit fixes three spelling typos in devlink-dpipe.rst and
devlink-port.rst.

Signed-off-by: Eva Dengler <eva.dengler@fau.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>