usbmon: Update pipe removal to suit my taste

This is a set of small updates to Alan's work to make the code more to
my liking. Mostly premature optimizations, but also direction of control
transfers in the binary interface was always out.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>