net: usb: rtl8150: demote allmulti message to dev_dbg()

This driver can spam the kernel log with multiple messages of:

    net eth0: eth0: allmulti set

Usually 4 or 8 at a time (probably because of using ConnMan).

This message doesn't seem useful, so let's demote it from dev_info()
to dev_dbg().

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: David S. Miller <davem@davemloft.net>