iwlwifi: fix default LQ table in 5.2 band

The default LQ is filled decreasingly using
iwl_get_prev_ieee_rate from a starting rate.
Since the starting rate is already the lowest one for
a specific band it should be actually filled evenly with
the starting rate: 1M and 6M for 5.2GHZ and 2.4GH respectively.
The bug is that for for A or G-only it decreases to
CCK rates which are not supported.
iwl_get_prev_ieee_rate function is just not band aware.
This affects broadcast station which lq table
is not updated by rs algorithm

G-only scenario is not treated by this patch

iwl_get_prev_ieee_rate is removed completely as it
is not used in other contexts

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>