staging: et131x: In et131x_tx(), don't return NETDEV_TX_BUSY, just drop the packet

Memory allocation errors do not denote NETDEV_TX_BUSY, simply drop
the packet silently with kfree_skb() and return NETDEV_TX_OK.

Also remove this item from the TODO list.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>