msm: mhi_dev: Avoiding ereq memory allocation in start

Start channel commands from MHI Host are given for all the set of valid
channels during the initialization itself. Previously we are allocating
ereq memory as part of start channel command. But, many of these channels
may not be opened by the clients and the ereq memory allocated is not
useful here.

Here avoiding ereq memory allocation during start channel command and
allocating memory only as part of open channel call by the clients. This
will reduce the overall memory usage and avoid the memory allocation
failures.

Change-Id: I60eccd1d17c60e1108820e1532f2ae385455f336
Signed-off-by: Sai Chaitanya Kaveti <quic_skaveti@quicinc.com>