usb: fix inflight cancel count stats, improve debug output
usb: fix inflight cancel count stats, improve debug output

This commit reworks the cancel counts, ensuring that they cannot
go negative. This validates that each xfer that is caused is also
counted and an attempt to cancel it is made when close() is called.

Closes #53.

Signed-off-by: Harry van Haaren <6e1296516ee88185cd60d9725251227ece1af74d@gmail.com>

openAVproductions/openAV-Ctlra