coresight-tmc-etr: use sysfs_buf in tmc_sg_get_rwp_offset()

When usb bypass start, etr_buf may be null. If etr disable hw is
called at the same time. Use sysfs_buf in tmc_sg_get_rwp_offset to
avoid this issue.

Change-Id: Id1e45269d08f61413d385de7aa46df4cc3d8c44d
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>