iwlwifi: remove shadow_reg_enable from hw_params

There's no need to copy shadow_reg_enable into
hw_params since it is a pure hardware parameter
that will never change, we can access it from
the config directly.

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>