net: mvneta: rename MVNETA_GMAC2_PSC_ENABLE to MVNETA_GMAC2_PCS_ENABLE

Bit 3 of the MVNETA_GMAC_CTRL_2 is actually used to enable the PCS,
not the PSC: there was a typo in the name of the define, which this
commit fixes.

Cc: stable@vger.kernel.org
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>