sdhci: Make sdhci_disable_irq_wakeups() static

sdhci_disable_irq_wakeups() is exported, but it is not called outside sdhci.c.

Make it static and do not export it, so that the following sparse warning is
fixed:

drivers/mmc/host/sdhci.c:2548:6: warning: symbol 'sdhci_disable_irq_wakeups' was not declared. Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>