msm: mhi_dev: Fix to avoid double free in alloc_evt_buf_evt_req()

There is a possible race condition, if the client calls
MHI close and channel gets a start command then
mhi_dev_alloc_evt_buf_evt_req() would be called. This can lead to
tr_events and ereqs getting freed from both the contexts.

Fix is to use a mutex to prevent this from happening parallelly.

Change-Id: I9ac10211d5bd5cee96e69b7fc10955e24569cf64
Signed-off-by: Subramanian Ananthanarayanan <skananth@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>