lantiq/xrx200-net: fix "tx ring full" error by introducing second DMA TX channel

With an own DMA TX channel for each network device (eth0 + eth1) there
won't be any "tx ring full" errors any more.

This patch also move the spinlocks to the channel level instead of locking
the whole xrx200_hw structure.

Signed-off-by: Martin Schiller <mschiller@tdt.de>