net: dsa: sja1105: Remove duplicate rgmii_pad_mii_tx from regs

The pad_mii_tx registers point to the same memory region but were
unused. So convert to using these for RGMII I/O cell configuration, as
they bear a shorter name.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>