Tegra2: mmc: Add data transfer completion timeout

Currently when no expected completion condition occures in the
mmc_send_cmd while loop that is waiting for a data transfer to
complete the MMC driver just hangs.

This patch adds an arbitrary 2 second timeout.  If nothing we
recognize occures within 2 seconds some diagnostic information
is printed and we fail out.

Signed-off-by: Anton Staaf <robotboy@chromium.org>
Cc: Andy Fleming <afleming@gmail.com>
Cc: Tom Warren <twarren@nvidia.com>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>