clocksource/imx-tpm: Correct -ETIME return condition check

The additional brakects added to tpm_set_next_event's return value
computation causes (int) forced type conversion NOT taking effect, and the
incorrect value return will cause various system timer issue, like RCU
stall etc..

Remove the additional brackets to make sure tpm_set_next_event always
returns correct value.

Fixes: 059ab7b8 ("clocksource/drivers/imx-tpm: Add imx tpm timer support")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Dong Aisheng <Aisheng.dong@nxp.com>
Cc: stable@vger.kernel.org
Cc: daniel.lezcano@linaro.org
Cc: Linux-imx@nxp.com
Link: https://lkml.kernel.org/r/1524117883-2484-1-git-send-email-Anson.Huang@nxp.com