Input: wm97xx-core - only schedule interrupt handler if not already scheduled

As well as clarifying the fact that the driver can cope if a second
interrupt occurs before the IRQ work is scheduled this also ensures
that calls to the machine irq_enable() are balanced, making that easier
to implement.  Normally this is redundant due to the interrupt disabling
but some unusal board configurations can trigger it.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>