mmc: sdhci-omap: Fix DCRC error handling during tuning

Commit 7d33c358 ("mmc: sdhci-omap: Workaround for Errata i802")
disabled DCRC interrupts during tuning. This write to the interrupt
enable register gets overwritten in sdhci_prepare_data() and the
interrupt is not in fact disabled. Fix this by disabling the interrupt
in the host->ier variable.

Fixes: 7d33c358 ("mmc: sdhci-omap: Workaround for Errata i802")
Cc: <stable@vger.kernel.org>
Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>