phy/marvell: fix 88e1121 support

Commit c477d044 added support for RGMII
rx/tx delays except that it ends up clearing rx/tx delays bit for modes
differents that RGMII*ID. Due to this, ethernet is not working anymore
on my guruplug server +. This patch is fixing that.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: David S. Miller <davem@davemloft.net>