gcc5 warns about using flags uninitialized in spin_lock_irqsave,
although it could look into the static inline spin_lock_irqsave
implementation and see it's not used at all. An empty define instead
of the static inline wrapper would lead to a "unused variable" warning.
Let's create a macro and fake some usage of the flags variable. This
probably helps until gcc6 is out.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>