mhi: core: Lock MHI_M3 state transition

To avoid any race conditions between MHI_M2 state
transition and MHI_M3 state transition lock the
entire MHI_M3 transition using xfer_lock.

CRs-Fixed: 972390
Change-Id: I7c2e1b7b3966dc5fb8bf2f91bce734bbc58c6fd7
Signed-off-by: Tony Truong <truong@codeaurora.org>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>