iwlwifi: make config const

Writing to the global config structures
is always wrong. To protect against such
mistakes in the future, mark them const.

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>