sdio: make the IRQ thread more resilient in the presence of bad states

Currently we print a message about some bad states wrt function IRQ
handlers but return 0 from process_sdio_pending_irqs() nevertheless.
This can lead to an infinite loop as nothing might have cleared the
condition for the pending card interrupt from the host controller by
the time host->ops->enable_sdio_irq(host, 1) is called.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>