coresight: trbe: Fix return value check in arm_trbe_register_coresight_cpu()

In case of error, the function devm_kasprintf() returns NULL
pointer not ERR_PTR(). The IS_ERR() test in the return value
check should be replaced with NULL test.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210409094901.1903622-1-weiyongjun1@huawei.com


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>