ath11k: Fix memory leak in ath11k_qmi_driver_event_work

The buffer pointed to by event is not freed in case
ATH11K_FLAG_UNREGISTERING bit is set, resulting in
memory leak, so fix it.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-01720.1-QCAHSPSWPL_V1_V2_SILICONZ_LITE-1

Fixes: d5c65159 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Signed-off-by: Baochen Qiang <bqiang@codeaurora.org>
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210913180246.193388-4-jouni@codeaurora.org