net: stmmac: dwmac-meson8b: use FIELD_PREP instead of open-coding it

Use FIELD_PREP() to shift a value to the correct offset based on a
bitmask instead of open-coding the logic.
No functional 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>