clocksource/drivers/pistachio: Correct output format of PTR_ERR()

Use signed integer output in the pr_err() string format, here PTR_ERR() value
is negative and it should be reported in human readable way.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Cc: Jisheng Zhang <jszhang@marvell.com>
Cc: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: http://lkml.kernel.org/r/1458603727-4446-1-git-send-email-vz@mleia.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>