net: macsec: Add missing documentation for 'gro_cells'

Fixes the following W=1 kernel build warning(s):

 drivers/net/macsec.c:113: warning: Function parameter or member 'gro_cells' not described in 'macsec_dev'
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201102114512.1062724-26-lee.jones@linaro.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/macsec.c

@@ -101,6 +101,7 @@ struct pcpu_secy_stats {
  * @real_dev: pointer to underlying netdevice
  * @stats: MACsec device stats
  * @secys: linked list of SecY's on the underlying device
+ * @gro_cells: pointer to the Generic Receive Offload cell
  * @offload: status of offloading on the MACsec device
  */
 struct macsec_dev {