net: fs_enet: mask interrupts for TX partial frames.

We are not interested in interrupts for partially transmitted frames.
Unlike SCC and FCC, the FEC doesn't handle the I bit in buffer
descriptors, instead it defines two interrupt bits, TXB and TXF.

We have to mask TXB in order to only get interrupts once the
frame is fully transmitted.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>