mfd: ezx-pcap: Remvove open coded irq handling

There is no point in checking irq_desc here, as it _is_ available. The
driver configured those lines, so they cannot go away.

The home brewn disabled/note_interrupt magic can be removed as well by
adding a irq_disable callback which avoids the lazy disable.

That driver needs to be converted to threaded interrupts.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>