ipc: icmsg: Check return error of pbuf_rx_init()

When pbuf_rx_init() was added, this caller did not check for a possibly
returned error to not have more overhead than before using this
function.
Although unlikely let's check for a possible error (not configured Rx
pbuf cfg).

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>