mmc: sdhci-acpi: Remove enable_dma() hook

This hook was solely used to set the DMA mask, which is now done
by the newly-added sdhci_set_dma_mask() function.

The use of a flag to ensure the mask is only set once is a strong hint
that it should not have been done there anyway.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>