tmc-etr: Add ETR status check in usb_notifier

It is possible that notify call is called after etr is disabled or
after mode switch from usb to etr. Add etr status check to avoid
exception.

Change-Id: I4feb36df28b6fc059598af98b6fb9ccf698d3aaf
Signed-off-by: Mao Jinlong <jinlmao@codeaurora.org>