handle MHI M1->M2 state transitions in qwx(4)

handle MHI M1->M2 state transitions in qwx(4)

When the device signals transition to state M1, the MHI driver may
set the device into M2 state to save power. The MHI device will be
woken up again by the driver ringing the wake doorbell register
before doing PCI reads/writes, which qwx(4) already implements.