coresight: tmc-etr: fix etr enable issue

Only usb_bam mode requires the bam related operation in function
_tmc_disable_etr_sink(), so adding a judgment to ensure this.

Need to wait for usb_read_work_fn() to complete, then drvdata->usb
can be assigned to NULL.

Change-Id: Ieb9a5ffb89be70cc3b14487c78a43b7aa31e90ec
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>