net: phy: marvell 88e151x: Fix handling of bare RGMII interface type

Commit 68e6ecad ("net: phy: marvell 88e151x: Fix handling of RGMII
interface types") fixed the initialization of 88e151x phys, but made it
so that interfaces of type PHY_INTERFACE_MODE_RGMII had both RX and TX
delay bits cleared. The default (like in m88e1111s_config) is to have
both bits set.

Hence, this patch changes the behavior in the PHY_INTERFACE_MODE_RGMII
case so that both bits are set.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>