clocksource/drivers/stm32: Fix error return code

Return an error code on failure.

Problem found using Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: kernel-janitors@vger.kernel.org
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lkml.kernel.org/r1528640655-18948-3-git-send-email-Julia.Lawall@lip6.fr