msm: mhi_dev: Using channel mutex lock for channel specific lists

Using channel mutex lock while accessing channel specific lists,
event_req_buffers and flush_event_req_buffers. This will avoid any
synchronization issues for these lists. Previously two different mutex
locks, global MHI lock and channel lock are used while accessing these
lists.

Removing MHI spin lock while accessing the above lists in
mhi_dev_process_reset_cmd, mhi_dev_queue_transfer_completion and
mhi_dev_flush_transfer_completion_events APIs as they are already
protected by channel mutex lock obtained from the APIs that are
calling these three APIs.

Removing MHI spin lock in mhi_dev_event_msi_cb and adding channel mutex
lock. In other instances where these lists are accessed they are already
protected by channel mutex lock.

Change-Id: I58b673be452a942ff946e07f17c96a0715805621
Signed-off-by: Sai Chaitanya Kaveti <quic_skaveti@quicinc.com>