tmc-etr: Fix duplicate logs issue when use sw usb mode

If etr driver data size is smaller than the sysfs buffer size,
it will read the data from the begainning of etr buffer repeatedly
when the actual read size plus the offset is greater than etr
driver data size. Compare with etr buffer size instead of etr
driver data size to fix this issue.

Change-Id: Ic523f8c5efd8348de7be80128a96d4e1ce867b9b
Signed-off-by: Mao Jinlong <jinlmao@codeaurora.org>