clocksource/drivers/moxart: Replace panic by pr_err

The clksrc-of code is supposed to catch the return code and fail gracefully.

Don't panic on error, but print the error and exit with a relevant error
code.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Joel Stanley <joel@jms.id.au>