net: marvell: mvpp2: fix AN restart

phylink already limits which interface modes are able to call the
MACs AN restart function, but in any case, the commentry seems
incorrect: the AN restart bit does not automatically clear when
set.  This has been found via manual setting using devmem2, and
we can observe that the AN does indeed restart and complete, yet
the AN restart bit remains set.  Explicitly clear the AN restart
bit.

Tested-by: Sven Auhagen <sven.auhagen@voleatech.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>