alc: Stablize this driver and fix various porting overlooks

alc: Stablize this driver and fix various porting overlooks

- Use ifnet serializer to protect driver data.  This should fix the
  various panics that were observed on the network output path.
- Fix busdma setup.
- Always allocate the descriptor rings and status blocks at the lower
  4GB.  The original way will require the RX/TX bufs to be located at
  the lower 4GB, which is a great waste of bounce buffers, cpu cycles
  and memory bandwidth.
- Don't mess up M_PKTHDR on RX path.
- Allocate mbuf w/ MB_WAIT on initialization path.
- Enable MSI by default.