iwlwifi: FW API: reference enum in docs of modify_mask

Add a reference to the correct enum rather than showing
the pattern of the actual constants.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>