iwlwifi: mvm: fix old scan version sizes

When version 8 of the scan command API was introduced, only the size
of version 7 was updated, causing older versions of the firmware to
throw BAD_COMMAND errors.

Calculating the old version based on the size of the latest version
got too complicated and the size of the older versions will never
change anyway, so it's better to just hardcoded the sizes.

Fixes: 66fa2424 ("iwlwifi: fw api: support the new scan request FW API version")
Reported-by: Scott Register <sreg@sreg.io>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>