mmc: sdhci: fix wakeup configuration

Activating wakeup event is not enough to get a wakeup signal. The
corresponding events have to be enabled in the Interrupt Status Enable
Register too. It follows the specification and is needed at least by
sdhci-of-at91.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>