mmc: sdhci: Rework sdhci_enable_irq_wakeups()

In preparation for adding more conditions for whether IRQ wakeup is
enabled, rework sdhci_enable_irq_wakeups() so that needed bits are added
instead of adding them all and then removing the unneeded bits.

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