clk: sunxi-ng: Make fractional helper less chatty

ccu_frac_helper_read_rate() prints some info which is not really
helpful except during debugging.

Replace printk() with pr_debug().

Fixes: 89a3dfb7 ("clk: sunxi-ng: Add fractional lib")
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>