genirq: Fix documentation of default chip disable()

The documentation says that by default disable() will be
chip->mask but in fact default_disable() is a noop.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
LKML-Reference: <1262698198-30392-1-git-send-email-broonie@opensource.wolfsonmicro.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>