soc: imx8m: Fix incorrect check for of_clk_get_by_name()

of_clk_get_by_name() returns error pointers instead of NULL.
Use IS_ERR() checks the return value to catch errors.

Fixes: 836fb309 ("soc: imx8m: Enable OCOTP clock before reading the register")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>