ath11k: fix resource leak in ath11k_mac_sta_state

Handled the error case with proper resource cleanup and
moved the handling into a separate function from
ath11k_mac_sta_state.

Signed-off-by: Karthikeyan Periyasamy <periyasa@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>