serial_lpuart: Fix config check issue when using clk driver in SPL

Should use CONFIG_IS_ENABLED not IS_ENABLED for CLK driver, so it will
check the CONFIG_SPL_CLK when building SPL

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
1 file changed