msm: mhi_dev: Fix memory leak during channel reset/stop

mhi_ring_start() does not check whether we once ever allocated DMA buffer
or not , causing it to leak memory if channel reset/stop happens.
Also removing the free logic in mhi_deinit() as the check would suffice
to prevent reallocation.

Change-Id: I569560f9de9facc690131c56f657eadb77a12811
Signed-off-by: Subramanian Ananthanarayanan <skananth@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>