powerpc/fm: fix TBI PHY address settings

TBI PHY address (TBIPA) register is set in general frame manager
phy init funciton dtsec_init_phy() in drivers/net/fm/eth.c, and
it is supposed to set TBIPA on FM1@DTSEC1 in case of FM1@DTSEC1
isn't used directly, which provides MDIO for other ports. So
following code is wrong in case of FM2, which has a different
mac base.

struct dtsec *regs = (struct dtsec *)fm_eth->mac->base;
/* Assign a Physical address to the TBI */
out_be32(&regs->tbipa, CONFIG_SYS_TBIPA_VALUE);

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>