mmc: usdhi6rol0: fix NULL pointer deref in debug print

host->sg is only set when we're transferring multiple blocks.  Check for
its availibility before dereferencing it in the timeout work debug
print.

Signed-off-by: Rabin Vincent <rabin.vincent@axis.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>