bonding: change AD_LINK_SPEED_BITMASK to enum to suport more speed

Port Key was determined as 16 bits according to the link speed,
duplex and user key (which is yet not supported).  In the old
speed field, 5 bits are for speed [1|10|100|1000|10000]Mbps as
below:
--------------------------------------------------------------
Port key :| User key        | Speed         |       Duplex|
--------------------------------------------------------------
    16                  6               1               0
This patch keeps the old layout, but changes AD_LINK_SPEED_BITMASK
from bit type to an enum type.  In this way, the speed field can
expand speed type from 5 to 32.

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