msm: mhi-dev: Check to avoid null pointer dereference

If mhi is not enabled the channels ring remain uninitialized. The host
might reset mhi which will result in device crash due to
null pointer dereference for the channel ring. Check if the pointer is
NULL before dereferencing it to avoid the crash.

Change-Id: I582f9a8a6c9bdca8d5f57e1800b566cca389b830
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>