Increase TX_MAX_SEGS from 10 to 20 for the if_awg.c driver

Under certain traffic pattern awg driver does not recover from TX queue
full condition. The actual source of the problem is not identified yet
but jmcneill@ agreed that bumping TX_MAX_SEGS to 20 is OK as a workaround
for the problem (NetBSD has it set to 128).

Also add some diagnostic printfs to prevent silent failure of bus_dma
functions in the future

PR will be kept open until root cause of the issue is identified and fixed

PR: 219927
Submitted by: Tom Vijlbrief <tvijlbrief@gmail.com>
Approved by: jmcneill
MFC after: 2 weeks