Just over a year ago (!) I had this brief exchange with Alan Stern:

>> It seems that the signal that can be used with USBDEVFS_DISCSIGNAL and 
>> in usbdevfs_urb.signr is limited to the real-time signals SIGRTMIN to 
>> SIGRTMAX. What's the rationale for this restriction? I believe that a 
>> process can kill() itself with any signal number, can't it? I was 
>> planning to use SIGIO for usbdevfs_urb.signr and SIGTERM (uncaught) for 
>> USBDEVFS_DISCSIGNAL. I don't think I'll have a problem with using 
>> SIGRTMIN+n instead, but I'm curious to know if there's some subtle 
>> problem with the non-real-time signals that I should be aware of.
>
> I don't know of any reason for this restriction.

Since no-one else could think of a reason either, I offer the following 
patch which allows any signal to be used with USBDEVFS_DISCSIGNAL and 
usbdevfs_urb.signr.
Signed-off-by: Phil Endecott <usbpatch@chezphil.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>