Expand tx dma maps to allow packets up to the MTU limit to be transmitted.

Expand tx dma maps to allow packets up to the MTU limit to be transmitted.
Don't clear TPB_TX_BUF_EN when taking the interface down, as somehow that
causes transmission to fail when the interface comes up again if any large
packets had been sent.

Handle packets received into multiple rx buffers using the same approach as
ixl(4), with the addition that a receive error can be signaled on any of
the buffers rather than just the last one.  Set the low water mark on the
rx ring so there are always enough buffers to receive the largest packet
supported by the interface, and fix the calculation of the data pointer
in rx buffers so there's actually MCLBYTES space available.

ok dlg@