clk: sunxi-ng: multiplier: Fix fractional mode

Driver for multiplier clock is missing a call to
ccu_frac_helper_enable() when fractional mode is selected.

Add a call to ccu_frac_helper_enable().

Fixes: d77e8135 ("clk: sunxi-ng: multiplier: Add fractional support")
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>