wifi: ath12k: Remove unused scan_flags from struct ath12k_wmi_scan_req_arg

As we did for ath11k lets remove the unused scan_flags from struct
ath12k_wmi_scan_req_arg. This will prevent us from using out of sync values
between WMI_SCAN_XXX & scan_f_xxx bitfield.
While at it remove the underlying wrapping struct/union construct as it serves
no purpose anymore.

Signed-off-by: Nicolas Escande <nico.escande@gmail.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://msgid.link/20240211145548.1939610-3-nico.escande@gmail.com