of_net: factor out repetitive code from of_get_mac_address()

of_get_mac_address() basically does the same  thing thrice, every  time with  a
different  property  name,  so it makes sense to factor out the repetitive code
into separate function. While at it, we can start using ETH_ALEN instead of the
bare number and drop unnecessary parens around the property length check.

The resulting ARM object file is 100 bytes less in size than before the patch.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>