mxc_nand: do not depend on disabling the irq in the interrupt handler

This patch reverts the driver to enabling/disabling the NFC interrupt
mask rather than enabling/disabling the system interrupt.  This cleans
up the driver so that it doesn't rely on interrupts being disabled
within the interrupt handler.

For i.MX21 we keep the current behaviour, that is calling
enable_irq/disable_irq_nosync to enable/disable interrupts.  This patch
is based on earlier work by John Ogness.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: John Ogness <john.ogness@linutronix.de>
Tested-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>