Increase the number of TX DMA segments from 32 to 35.  It turned
out 32 is not enough to support a full sized TSO packet.
While I'm here fix a long standing bug introduced in r169632 in
bce(4) where it didn't include L2 header length of TSO packet in
the maximum DMA segment size calculation.

In collaboration with:	rmacklem
MFC after:		2 weeks