netxen: optimize tx handling

netxen driver allows limited number of threads simultaneously posting
skb's in tx ring. If transmit slot is unavailable, driver calls
schedule() or loops in xmit_frame().

This patch returns TX_BUSY and lets the stack reschedule the packet if
transmit slot is unavailable. Also removes unnecessary check for tx
timeout in the driver itself, the network stack does that anyway.

Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>