MPC5XXX, Motion-PRO: Fix PHY initialization problem.

After being reset in mpc5xxx_fec_init_phy(), PHY goes into FX mode, in which
networking does not function. This commit switches PHY to TX mode by clearing
the FX_SEL bit of Mode Control Register. It also reverses commit
008861a2, i.e., a temporary workaround.

Signed-off-by: Grzegorz Bernacki <gjb@semihalf.com>
Acked-by: Bartlomiej Sieka <tur@semihalf.com>