clk: sunxi-ng: nm: Check if requested rate is supported by fractional clock

The round_rate callback for N-M-factor style clocks does not check if
the requested clock rate is supported by the fractional clock mode.
While this doesn't affect usage in practice, since the clock rates
are also supported through N-M factors, it does not match the set_rate
code.

Add a check to the round_rate callback so it matches the set_rate
callback.

Fixes: 6174a1e2 ("clk: sunxi-ng: Add N-M-factor clock support")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>