Add the missing structure field `rx_len` description.

Link: https://lore.kernel.org/r/20220817172731.1185305-2-cristian.marussi@arm.comSigned-off-by: NCristian Marussi <cristian.marussi@arm.com>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>