genirq: Correct fuzzy and fragile IRQ_RETVAL() definition

commit bedd30d9 ("genirq: make irqreturn_t an enum") blindly replaced
"0" by "IRQ_NONE" in the "IRQ_RETVAL(x)" macro definition.

However, as "x" is a condition, "0" meant "boolean false", not an
irqreturn_t value.

All of this worked, and kept working after the addition of IRQ_WAKE_THREAD,
as
  - both "boolean false" and "IRQ_NONE" are "0" (for the comparison),
  - "boolean true" and "boolean false" nicely map to the correct values of
    "IRQ_HANDLED" and "IRQ_NONE" (for the return value).

Correct the macro definition for clarity and future-proofness.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>