net: fs_enet: explicitly remove I flag on TX partial frames

We are not interested in interrupts for partially transmitted frames,
we have to clear BD_ENET_TX_INTR explicitly otherwise it may remain
from a previously used descriptor.

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