qtnfmac: fix resource leaks on unsupported iftype error return path

Currently if an unsupported iftype is detected the error return path
does not free the cmd_skb leading to a resource leak. Fix this by
free'ing cmd_skb.

Addresses-Coverity: ("Resource leak")
Fixes: 805b28c0 ("qtnfmac: prepare for AP_VLAN interface type support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200925132224.21638-1-colin.king@canonical.com