net: stmmac: dwmac-meson8b: fix enabling the timing-adjustment clock

The timing-adjustment clock only has to be enabled when a) there is a
2ns RX delay configured using device-tree and b) the phy-mode indicates
that the RX delay should be enabled.

Only enable the RX delay if both are true, instead of (by accident) also
enabling it when there's the 2ns RX delay configured but the phy-mode
incicates that the RX delay is not used.

Fixes: 9308c476 ("net: stmmac: dwmac-meson8b: add support for the RX delay configuration")
Reported-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>