et131x: Fix logical vs bitwise check in et131x_tx_timeout()

We should be using a logical check here instead of a bitwise operation
to check if the device is closed already in et131x_tx_timeout().

Reported-by: coverity (CID 146498)
Fixes: 38df6492 ("et131x: Add PCIe gigabit ethernet driver et131x to drivers/net")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>