coresight: ultrasoc-smb: fix return value check in smb_init_data_buffer()

platform_get_resource() returns NULL pointer not PTR_ERR(), replace
the IS_ERR() check with NULL pointer check.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20230118074920.1772141-1-yangyingliang@huawei.com