net: stmmac: dwmac-meson8b: Move the documentation for the TX delay

Move the documentation for the TX delay above the PRG_ETH0_TXDLY_MASK
definition. Future commits will add more registers also with
documentation above their register bit definitions. Move the existing
comment so it will be consistent with the upcoming changes.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>