clocksource/drivers/mips-gic-timer: Fix an error code in 'gic_clocksource_of_init()'

'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>