[POWERPC] pasemi: clean up mdio_gpio a bit

Misc cleanups of mdio_gpio:
* Better error handling/unrolling in case of init/alloc failures
* Go through child nodes and get their interrupts instead of using
  hardcoded values
* Remap the GPIO registers at module load/driver init instead of during probe
* Coding style and other misc cleanups

Signed-off-by: Olof Johansson <olof@lixom.net>