msm: adsprpc: initialize async job member of context structure

Currently, the async job member of the ioctl structure is not
being initialized properly while making an unmap call to the
remote subsystem because of which an indeterminate value is used
to incorrectly mark this RPC call as an asynchronous job.
Initialize the async job field to NULL before making the RPC call
to unmap memory on DSP.

Change-Id: I611e9419793da32f73b9997c7dbe50e3d1e7e81b
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>
Signed-off-by: Himateja Reddy <hmreddy@codeaurora.org>