Allow sending of more than one raw cell from a single mbuf. Only the

Allow sending of more than one raw cell from a single mbuf. Only the
very first cell in the mbuf should have a cell header word (of which
everything except the payload type and the CLP bit is ignored). All
other cells should be 48 byte and get the same header as the first cell.

This fixes a problem with sending more than 120000 raw cells/sec through
an HE155. The card seems to need 2 cell times to DMA the transmit buffer
ready queue entry and the transmit buffer descriptor so at 1/3 the
link rate the transmit buffer ready queue starts to fill up. Even with this
patch it's obviously impossible to send raw cells at link rate.