clk: sunxi-ng: Fix fractional mode for N-M clocks

N-M factor clock driver is missing a call to ccu_frac_helper_enable()
when fractional mode is used. Additionally, most SoCs require that M
factor must be set to 0 when fractional mode is used.

Without this patch, clock keeps the old value and clk_set_rate() returns
without error.

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