DWC OTG TX path optimisation for 11-current

[Svatopluk Kraus]

Hi,

I finally found a way how to get something what could be analyzed
considering the problem with smcs warnings, slow console response, and
other oddities. And it was Michal Meloun who figured out how the
problem comes up. He was so enthusiastic and fast that he got me no
chance, no chance at all to figure it out myself. ;)) Thanks goes to
Bob Prohaska too who pointed me to stress2 which triggers the problem
even with all KTR classes compiled in kernel.

The fix is here: https://reviews.freebsd.org/D3722

Svata