iwlwifi: keep plcp_delta_threshold in priv

The base_params shouldn't be writable, so keep
a copy of this in priv that can be modified.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>