sh_eth: read MAC address registers only once

The code reading the MAHR/MALR registers in read_mac_address() is terribly
ineffective -- it reads MAHR 4 times and MALR 2 times, while it's enough to
read each register only once.  Use the local variables to achieve that,
somewhat beautifying the code while at it...

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