ath11k: unlock on error path in ath11k_mac_op_add_interface()

These error paths need to drop the &ar->conf_mutex before returning.

Fixes: 690ace20 ("ath11k: peer delete synchronization with firmware")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/X85sVGVP/0XvlrEJ@mwanda