net: phy: marvell10g: Use linkmode_set_bit helper instead of __set_bit

Cosmetic patch making use of helpers dedicated to linkmodes handling.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>