bonding: make USES_PRIMARY inline functions

Change the name a bit to better reflect its scope, and update some
comments. Two functions added - one which takes bond as a param and the
other which takes the mode.

CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>