ath10k: remove unnecessary code

The array fields in struct wmi_start_scan_arg that are checked here are
fixed size arrays so they can never be NULL.

Addresses-Coverity-ID: 1260031
Cc: Arend Van Spriel <arend.vanspriel@broadcom.com>
Cc: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>