Add reset timer, fire it 1/2 sec after 'Too much work in interrupt'
Move reset code from tx_timeout into two separate routines:
reset_and_disable_rxtx() and enable_rxtx()
New function reset_tx_descriptors(): clean up tx ring
after tx_timeout. tx_timeout now does:
    reset_and_disable_rxtx()
    reset_tx_descriptors()
    enable_rxtx()
    netif_wake_queue()
Absense of netif_wake_queue() call was probably the cause of
tx_timeout() stalling all future tx.
Remove stop_nic_tx(), not used anymore