net: arc_emac: reset txbd_curr and txbd_dirty pointers to zero

EMAC reset internal tx ring pointer to zero at statup.
txbd_curr and txbd_dirty can be different from zero.
That cause ethernet transfer hang (no packets transmitted).

In order to reproduce, run on device:
    ifconfig eth0 down
    ifconfig eth0 up

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>