tty: serial: 8250-of: Do not warn on deferred probe

Deferred probe is an expected return value for clk_get() on many
platforms. The driver deals with it properly, so there's no need
to output a warning that may potentially confuse users.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>