coresight: tmc-etr: fix null ptr dereferencing in usb_qdss_close

"sysfs_buf" will not be free while out_mode switching from mem to usb
and "usbch" will not be assigned in tmc_enable_etr_sink_sysfs, this
will cause passing null ptr to usb_qdss_close when switch out_mode
to mem.

Change-Id: Ibd8b5c78fb55231e34e16a9c32d43c2e771c0f30
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>