msm: adsprpc: Allow zero length buffer

Initialize the arguments being passed to account for zero
length buffer being passed and not invoke flush routines
in these cases.

Change-Id: Ic6b3418f05ea3ca46e7ca758ebbd0a7bda987b51
Acked-by: Sathish Ambley <sambley@qualcomm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Ram Kumar Chakravarthy Chebathini <rcheba@codeaurora.org>