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: Ic8e095bdc59cb45c618897c6db5f512245d450dd
Acked-by: Sathish Ambley <sambley@qualcomm.com>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>