ath10k: free wmi mgmt event skb when parsing fails

When wmi mgmt event function fails to parse given skb,
it should be freed on failure condition to avoid memory
leaks. Found this during the code review.

Signed-off-by: Raja Mani <rmani@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>