net: ethernet: ti: am65-cpts: Document am65_cpts_rx_enable()'s 'en' parameter

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

 drivers/net/ethernet/ti/am65-cpts.c:736: warning: Function parameter or member 'en' not described in 'am65_cpts_rx_enable'
 drivers/net/ethernet/ti/am65-cpts.c:736: warning: Excess function parameter 'skb' description in 'am65_cpts_rx_enable'

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>