ath6kl: wmi: Replace one-element array with flexible-array member in struct wmi_start_scan_cmd

Replace one-element array with flexible-array member in struct
wmi_start_scan_cmd. Also, make use of the struct_size() helper.

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/79


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/8b33c6d86a6bd40b5688cf118b4b35850db8d8c7.1645736204.git.gustavoars@kernel.org