libertas: don't exit from lbs_ibss_join_existing() with RCU read lock held

Commit e5e884b4 ("libertas: Fix two buffer overflows at parsing bss
descriptor") introduced a bounds check on the number of supplied rates to
lbs_ibss_join_existing().

Unfortunately, it introduced a return path from within a RCU read side
critical section without a corresponding rcu_read_unlock(). Fix this.

Fixes: e5e884b4 ("libertas: Fix two buffer overflows at parsing bss descriptor")
Signed-off-by: Nicolai Stange <nstange@suse.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>