bgmac: add helper checking for BCM4707 / BCM53018 chip id

Chipsets with BCM4707 / BCM53018 ID require special handling at a few
places in the code. It's likely there will be more IDs to check in the
future. To simplify it add this trivial helper.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>