mmc: omap: Remove always set use_dma flag from struct mmc_omap_host

Because use_dma is set only in mmc_omap_probe and unset nowhere there is no
need to carry that flag in struct mmc_omap_host for mmc_omap_prepare_data
function.

Signed-off-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Chris Ball <chris@printf.net>