sd/sdhci.c: Only reset data_count on new commands

sd/sdhci.c: Only reset data_count on new commands

The data_count variable was being reset on every transfer, including
DMA transfer resumptions. This is incorrect, it should only be set
on a new command.

Manifests as a bug when using ADMA and there is a timer delay between
ADMA frames where the fifo is left in a non empty state.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Igor Mitsyanko <i.mitsyanko@gmail.com>
Message-id: 15a98609cc32315211b0963091a8efd67522e160.1369370934.git.peter.crosthwaite@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>