net: netsec: initialize tx ring on ndo_open

Since we changed the Tx ring handling and now depends on bit31 to figure
out the owner of the descriptor, we should initialize this every time
the device goes down-up instead of doing it once on driver init. If the
value is not correctly initialized the device won't have any available
descriptors

Changes since v1:
- Typo fixes

Fixes: 35e07d23

 ("net: socionext: remove mmio reads on Tx")
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>