coresight-tmc: Adds lock to avoid race condition

There can be a case of race in the call to tmc_etr_bam_disable(). The call
from usb_notifier() to tmc_etr_bam_disable() is within a lock
'drvdata->mem_lock' but the call from tmc_disable is not under any lock
which can lead to race.

This patch adds the missing lock.

CRs-Fixed: 2047196
Change-Id: I3986b498ae8e2b2ddfe8f5b3315fdefab197d11c
Signed-off-by: Mukesh Ojha <mojha@codeaurora.org>