USB: Fix signr comment in usbdevice_fs.h

This trivial documentation patch corrects a comment in usbdevice_fs.h; it
previously suggested that the signal would only be sent on error, but I am
told that it is sent on both successful and unsuccessful completion, and
that zero indicates that no signal should be sent.

Signed-off-by: Phil Endecott <spam_from_usb_devel@chezphil.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>