clk: si5351: Fix clkout rate computation.

Rate was incorrectly computed because we read from wrong divider register.

Signed-off-by: Marek Belisko <marek.belisko@streamunlimited.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>
Cc: stable@kernel.org