backfire: Generic: Clean up output of AR8216 driver. (backport of r20773)

backfire: Generic: Clean up output of AR8216 driver. (backport of r20773)

This patch is rather a cosmetic patch. It fixes the following issues:

 * Demote the unknown device message to debug level to not spam
   the log.
 * Fix the version print of the unknown device message.
 * Output the 'attach' message only when attaching as switch driver,
   not when attaching as phy driver.
 * Correctly return NET_RX_DROP when dropping packets.

Signed-off-by: Jonas Gorski <jonas.gorski+openwrt@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/branches/backfire@20774 3c298f89-4303-0410-b956-a3cf2f4a3e73