fix(rss): fix msg deserialization bugs in comms

-fix1: size of struct instead of pointer during reply_size check
-fix2: update the out_vec length with the actual length from reply
       message (e.g. in case of an output buffer, the returned output
       data length remained the size of the buffer and was not updated
       with the size of the actual data in it)

Change-Id: Ibed5520ca1fb05df358de4bdf85ace219183866c
Signed-off-by: David Vincze <david.vincze@arm.com>