mhi: core: block unbind from userspace till channel reset

MHI clients can unbind driver from userspace while a channel
is active with ongoing transfers. As we can only do host side
clean-up of the channel without sending a reset channel command
to the device, a kernel panic could occur due to incoming
packets if we proceed. Wait until the client unprepares the
channel or device goes to an error state.

Change-Id: I400cbf799d96fb7bc05df4c568593e12e9dc41f1
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>