ath10k: add missing mutex unlock on failpath

Kernel would complain about leaving a held lock
after going back to userspace and would
subsequently deadlock.

Fixes: e04cafbc

 ("ath10k: fix peer limit enforcement")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>