UPSTREAM: net: arc_emac: reset txbd_curr and txbd_dirty pointers to zero

UPSTREAM: 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

Change-Id: Ie37cbd4761f8df216b2ddc5d07c0d6f036ac7092
Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
(cherry picked from git.kernel.org next/linux-next.git master
 commit 99f93a156a2aa9ac1e44f7cb1a197425e6d9879a)