clk: keystone: pll: fix a couple NULL vs IS_ERR() checks

The clk_register_divider() and clk_register_mux() functions returns
error pointers on error but this code checks for NULL.  Fix that.

Fixes: b9e0d40c ("clk: keystone: add Keystone PLL clock driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/d9da4c97-0da9-499f-9a21-1f8e3f148dc1@moroto.mountain


Signed-off-by: Stephen Boyd <sboyd@kernel.org>