drivers: clk: keystone: Fix parameter judgment in _of_pll_clk_init()

The function clk_register_pll() may return NULL or an ERR_PTR. Don't
treat an ERR_PTR as valid.

Signed-off-by: Minjie Du <duminjie@vivo.com>
Link: https://lore.kernel.org/r/20230712102246.10348-1-duminjie@vivo.com
Fixes: b9e0d40c

 ("clk: keystone: add Keystone PLL clock driver")
[sboyd@kernel.org: Reword commit text]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>