clocksource/drivers/em_sti: Fix error return codes in em_sti_probe()

Propagate the return values of platform_get_irq and devm_request_irq on
failure.

Cc: Frans Klaver <fransklaver@gmail.com>
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>