iwlwifi: change iwl_enable/disable_interrupts to "inline"

iwl_enable_interrupts is being called inside the interrupt,
change from function call to inline

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>