clk: meson: add some error handling in meson_clk_register_cpu()

This error handling hopefully isn't needed but it make the static
checkers happy.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>