virtio_ring: break out vring descriptor setup code

Break out the vring descriptor setup into a separate inline
function. No functional change. This will be used later to
deal with potential non-linear addresses used by virtio-rpmsg.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
[s-anna@ti.com: fixup flags type, revise commit log]
Signed-off-by: Suman Anna <s-anna@ti.com>