clocksource/drivers/timer-of: Checking for IS_ERR() instead of NULL

The current code checks the return value of the of_io_request_and_map()
function as it was returning a NULL pointer in case of error.

However, it returns an error code encoded in the pointer return value, not a
NULL value. Fix this by checking the returned pointer against IS_ERR() and
return the error with PTR_ERR().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>