msm: qdss: use spinlock instead of mutex for the etb driver

In preparation for subsequent changes that will provide an api
path into the etb driver which can be called from atomic context,
change mutex based mutual exclusion to spinlock.

Change-Id: I40c91124933b30ca8c5db6aa4c77a0f21884ccf9
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>