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.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>