rpmsg: virtio_rpmsg_bus: fix export of rpmsg_send_offchannel_raw()

Commit 8a228ecf

 ("rpmsg: Indirection table for rpmsg_endpoint
operations") has made the rpmsg_send_offchannel_raw() a static
function and local to the virtio_rpmsg_bus module, but has not
dropped the corresponding EXPORT_SYMBOL. Fix this.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>