of_mdio: merge branch tails in of_phy_register_fixed_link()

Looks  like gcc isn't always able to figure  out that 3 *if* branches in
of_phy_register_fixed_link() calling fixed_phy_register() at their ends
are similar enough and thus can be merged. The "manual" merge saves 40
bytes of the object code (AArch64 gcc 4.8.5), and still saves 12 bytes
even  if gcc was able to merge the branch tails (ARM gcc 4.8.5)...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>