msm: mhi_dev: add WQ_UNBOUND and set max_active for mhi_sm_wq

Workqueue tasks can be executed in the order in which
they are submitted using a WQ_UNBOUND workqueue with
max_active set to one. Adding these properties for mhi_sm_wq
so that all MHI change events are handled in order.

Change-Id: Ic86a81adb379f711d787cd40dfad4a4a8f6e961d
Signed-off-by: Nitesh Gupta <nitegupt@codeaurora.org>