coresight-tmc: Use sysfs_buf for byte-cntr and sw usb cases

Etr_buf will be NULL when disable etr hw. It is before stopping byte-cntr
and sw usb data transfer which may cause null pointer issue. Use sysfs_buf
to avoid this issue.

Change-Id: I5967fd4183a9f36fca22059bb08fd8d357c8e5af
Signed-off-by: Mao Jinlong <jinlmao@codeaurora.org>