DWC OTG TX path optimisation for 11-current

[Hans Petter Selasky]

Hi,

I did some testing myself and I see the polling handler can loop many 
times when USB is active. Instead of 2 as a polling limit I've set 16. 
Hope this works too. Works fine over here.

https://svnweb.freebsd.org/changeset/base/286118

--HPS