'clk' is a valid pointer at this point. So calling PTR_ERR on it is
pointess.

Return the error code from 'clk_prepare_enable()' if it fails instead.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>