phy: marvell: a3700: Fix configuring polarity invert bits

phy_txd_inv or phy_rxd_inv needs to be set only in case when
appropriate polarity is inverted. Otherwise these bits should be
cleared.

Same change was included in TF-A project:
https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/9406



Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>