round_down() can only round to powers of 2. If round_down() is asked
to round to something that is not a power of 2, incorrect results are
produced. The incorrect results can be both too large and too small.

Instead, use rounddown() which can round to any number.

Fixes: 6a721db1 ("clk: sunxi: Add A31 clocks support")
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>