[TG3]: nvram cleanup

Some nvram related cleanup:

1. Add a tg3_nvram_read_swab() since swabing the data is frequently
done.

2. Add a function to convert nvram address to physical address
instead of doing it in 2 separate places.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>