stmmac: prevent false carrier sense detection

This patch is to w/a a problem that happens on some boxes when run at 10Mbps
Half duplex mode.

During the transmission the CSR signal is asserted for some time and the frames
aborted because of carrier sense error.
This is reported by MMC HW counter: txcarrier signal.
This actually is a false carrier so the frames are good and there is no reason
to ask for dropping them.

This patch so disables the Carrier Sense During Transmission
and this means that the MAC transmitter ignore the CRS signal
during frame transmission in Half-Duplex mode.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Acked-by: Vince Bridgers <vbridgers2013@gmail.com>
Acked-by: Ley Foon Tan <lftan@altera.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>