mmc: sunxi: Check the value returned by clk_round_rate

clk_round_rate() may return an error. Check it.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>