bus: mhi: host: Fix race between channel preparation and M0 event

There is a race condition where mhi_prepare_channel() updates the
read and write pointers as the base address and in parallel, if
an M0 transition occurs, the tasklet goes ahead and rings
doorbells for all channels with a delta in TRE rings assuming
they are already enabled. This causes a null pointer access. Fix
it by adding a channel enabled check before ringing channel
doorbells.

Cc: stable@vger.kernel.org # 5.19
Fixes: a6e2e352

 "bus: mhi: core: Add support for PM state transitions"
Signed-off-by: Qiang Yu <quic_qianyu@quicinc.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://lore.kernel.org/r/1665889532-13634-1-git-send-email-quic_qianyu@quicinc.com


[mani: CCed stable list]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>