mmc: sdhci: Handle failure of enable_irq_wake()

Now that sdhci_enable_irq_wakeups() is a local function, change it to
return whether the IRQ wakeup was successfully enabled. This is in
preparation for adding more conditions for whether IRQ wakeup is enabled.

Note it is assumed, for SDHCI devices, that suspend is more important than
wakeup, so we continue to suspend regardless.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>