ravb: read MAC address registers only once

The code reading the MAHR/MALR registers in ravb_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>