bus: mhi: ep: set devices to NULL in mhi_ep_destroy_device

The driver sets per-channel device pointers to NULL only in the release
callback. However this does not seem correct. After the put_device() is
called, the pointer is dangling, so it's better to set it to NULL right
on the spot.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>