iwlwifi: move IWL_MASK into file using it

Only used in two places in the same file,
no need to be in iwl-shared.h.

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>