coresight: tnoc: Fix a NULL vs IS_ERR() bug in probe

The devm_ioremap_resource() function returns error pointers on error.
It never returns NULL.  Update the error checking to match.

Fixes: e54a52a2 ("coresight: add coresight Trace Network On Chip driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/7bd9fae8-a15f-412a-8800-ce47acf0b5ce@sabinyo.mountain