clk: rockchip: rk3399: fix up the make warning

fix up the warning:
Line 246: 			rate_table->nf = nf_out;
drivers/clk/rockchip/clk-pll.c:246:19: warning: 'nf_out' may be used uninitialized in this function [-Wmaybe-uninitialized]

Change-Id: I6c11bf91a280f324c21214cb7839e36f9ffa0da3
Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>

drivers/clk/rockchip/clk-pll.c

@@ -191,6 +191,7 @@ rockchip_rk3066_pll_clk_set_by_auto(struct rockchip_clk_pll *pll,
 
 	nr_out = PLL_NR_MAX + 1;
 	no_out = 0;
+	nf_out = 0;
 
 	if (fin_hz == 0 || fout_hz == 0 || fout_hz == fin_hz)
 		return NULL;