Fix rpmsg not being zero initialized

The rpmsg struct used in response messages is not zero initialized so
later response messages may contain old data. To resolve this, the rpmsg
struct is now zero initialized for each new message.

Change-Id: I116fa443eca7b6ec3d8817909fd3c59bd69427cb
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>