net: mvpp2: set mac address does not require the stop/start sequence

Remove special stop/start handling from the set_mac_address callback.
All this special care is not needed, and can be removed. It also
simplifies the up/down status in the driver and helps avoiding possible
link status mismatch issues.

Signed-off-by: Yan Markman <ymarkman@marvell.com>
[Antoine: commit message]
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>