iwlwifi: add rs_rate_scale_clear_tbl_windows helper function

instead of duplicating the same loop multiple times,
use a new function for it.

this will be later used also for clearing other
windows in the table.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>