Linux: Fix warnings seen when compiling with gcc-4.6

Linux: Fix warnings seen when compiling with gcc-4.6

Compiling with gcc-4.6 results in the following warnings:
os/linux_usbfs.c: In function 'op_get_configuration':
os/linux_usbfs.c:1144:6: warning: variable 'r' set but not used ...
os/linux_usbfs.c: In function 'op_handle_events':
os/linux_usbfs.c:2091:40: warning: 'status' may be used uninitialized ...
os/linux_usbfs.c:2044:6: note: 'status' was declared here
dpfp.c: In function 'save_to_file':
dpfp.c:228:9: warning: variable 'ignore' set but not used ...
dpfp_threaded.c: In function 'save_to_file':
dpfp_threaded.c:257:9: warning: variable 'ignore' set but not used ...

This patch fixes these. (The second comes from handle_control_completion()
which gcc seems to inline into op_handle_events().)

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[stuge: Add fixes for dpfp examples and update source references]