staging: wfx: fix RCU usage between hif_join() and ieee80211_bss_get_ie()

staging: wfx: fix RCU usage between hif_join() and ieee80211_bss_get_ie()

Access to result of ieee80211_bss_get_ie() is protected by RCU. In other
hand, function hif_join() can sleep and cannot be called with RCU
locked.

Provide a copy of "ssidie" to hif_join() to solve this behavior.

Fixes: 9ced9b593741 ("staging: wfx: simplify hif_join()")
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200310101356.182818-6-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>