msm: qdss: initialize mutex earlier in etb_probe

Initialize mutex earlier in etb_probe to allow for it to be
used by subsequent code if required. Make corresponding changes
to mutex destroy for the error path as well as etb_remove.

Change-Id: Iac3c4fe9ddb0f31d9a0faaca3f526e928e35f6cc
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>