msm: adsprpc: Fix build issue due to casting from large to small type

Fix compilation issue while Casting a large number type to a smaller type
which may lead to data lost.

Replace ‘null’ with ‘0’ as null is 64bit wide.

Change-Id: Ic77dfac7c920cf3dbdfe459ec435680cb69be2f5
Signed-off-by: Vinay Sudra <vsudra@codeaurora.org>