net: phy: marvell: replace phy_modify()

a set of phy_set_bits() looks more neater

Signed-off-by: Yejune Deng <yejune.deng@gmail.com>
Link: https://lore.kernel.org/r/1606732895-9136-1-git-send-email-yejune.deng@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>