mmc: sdhci: further code simplication

Further simplify the code in sdhci_prepare_data() - we don't set
SDHCI_REQ_USE_DMA anywhere else in the driver, so there is no
need to set it, and then immediately test it.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>