msm: mhi_dev: Prevent invalid memory access during channel read

ring_cache is NULL as channel has not yet been started, read operation
accesses ring_cache before checking if channel is in stopped state.
Adding a check in UCI layer to perform read/write transfers only
if the channel is in valid state.

Change-Id: Ic8e6ae19badf43a7f64f92349e9c2427750a98b6
Signed-off-by: Subramanian Ananthanarayanan <skananth@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>