enumeration failure of disks in external box redux

[Gary Jennejohn]

I spent most of the day doing a binary search to find out which
commit broke enumeration of the disks in my external disk box,
which is using the JMicron USB3 bridge chip.

It turned out that there was some kind of breakage in my
/usr/src, which I verified by exporting r279520 to
/usr/src_279520 and then compiling and installing a new kernel
from there.  This kernel then worked correctly.

BUT!  I *must* set dma_bits in xhci.c to 32, otherwise
enumeration fails miserably.

I suspect that the problem reported recently in a 10.1 (?) PR is
also due to dma_bits being set to 64, since IIRC the reporter
has the same JMicron USB3 bridge chip in his box.

So, HPS, how about an option or quirk to force dma_bits to 32 for
us poor users stuck with this brain-damaged JMicron chip?

-- 
Gary Jennejohn