mhi: core: allow setting wake db only if we're on MHI M2 state

WAKE doorbell used for exiting MHI low power mode M2. To prevent
unnecessary interrupts on device side, only allow setting the
register if device is in M2 state

CRs-Fixed: 2248650
Change-Id: If84f57ceeae2ea17e32d2510a7d1da3a6a70c69b
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>