net: mvpp2: do not select the internal source clock

This patch stops the internal MAC Tx clock from being enabled as the
internal clock isn't used. The definition used for the bit controlling
this behaviour is renamed as well as it was wrongly named (bit 4 of
GMAC_CTRL_2_REG).

Fixes: 3919357f

 ("net: mvpp2: initialize the GMAC when using a port")
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>