mmc: moxart: fix wait_for_completion_interruptible_timeout return variable type

wait_for_completion_timeout_interruptible returns long not unsigned long
so dma_time, which is used exclusively here, is changed to long.

Fixes: 1b66e94e

 ("mmc: moxart: Add MOXA ART SD/MMC driver")
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>