msm: mhi_dev: Fix memory leak

mhi_dev_cache_host_cfg will be called during cold boot and also
when processing mhi device reset command from host. Do not re-allocate
the command, event and channel context cache memory if they have been
already allocated.

Change-Id: I55826e05b3d83c9fcf3d4b23e26b91253130671e
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>
Signed-off-by: Gauri Joshi <gaurjosh@codeaurora.org>