of/mdio: Fix some endianness problems.

In of_mdiobus_register(), the __be32 *addr variable is dereferenced.
This will not work on little-endian targets.  Also since it is
unsigned, checking for less than zero is redundant.

Fix these two issues.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
[grant.likely@secretlab.ca: removed goto]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>