bridge: leave TX queue length as kernel default, not 0

bridge: leave TX queue length as kernel default, not 0

Not using NewLinkAttrs() or not initializing TxQLen leaves
the value as 0, which tells the kernel to set a zero-length
tx_queue_len.  That messes up FIFO traffic shapers (like pfifo)
that use the device TX queue length as the default packet
limit.  This leads to a default packet limit of 0, which drops
all packets.