Replace one-element array with flexible-array member in struct
wmi_disconnect_event.

This issue was found with the help of Coccinelle and audited and fixed,
manually.

Link: https://www.kernel.org/doc/html/v5.16/process/deprecated.html#zero-length-and-one-element-arrays
Link: https://github.com/KSPP/linux/issues/79Reviewed-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/e9957dc53ae48c2f39ae57b0157a67d844b5bc20.1645736204.git.gustavoars@kernel.org