Clear MDC bits before setting them to a new value.  Adjust MDC value
for 10G.
Signed-off-by: NSweta Bhatt <sweta.bhatt@einfochips.com>
Signed-off-by: NChien Tung <ctung@neteffect.com>
Signed-off-by: NRoland Dreier <rolandd@cisco.com>