net: arc_emac: fix sk_buff leak

EMAC could be disabled, while there is some sb_buff
in use. That buffers got lost for linux.

In order to reproduce run on device during active ethernet work:
    ifconfig eth0 down

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