powerpc/mpc5200: Refactor FEC mdio read/write routines

Read & write functions now call a generic transfer function, so identical
code in both routines could be eliminated. The result is easier to maintain
and smaller in source and binary code. Also, fix some checkpatch warnings.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>