mmc: tmio: prevent endless loop in tmio_mmc_set_clock()

I spent a couple of days with the driver just hanging due to me forgetting to
specify the external crystal frequency,  so that clk_get_rate() returned 0 and
thus the loop in tmio_mmc_set_clock() never ended. I don't think that's an
acceptable behavior, so I suggest that the minimum frequency is checked for 0
in tmio_mmc_host_probe().

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Ian Molton <ian.molton@codethink.co.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>