staging: wfx: drop useless struct hif_join_flags

Struct hif_join_flags has no reason to exist. Drop it and simplify
access to struct hif_req_join.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200907101521.66082-13-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>