mmc: sdhci: Allow for irq being shared

If the SDHCI irq is shared with another device then the interrupt
handler can get called while SDHCI is runtime suspended.  That is
harmless but the warning message is not useful so remove it.  Also
returning IRQ_NONE is more appropriate.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <chris@printf.net>