USB: Remove unneeded void * casts in idmouse.c

The patch removes unneeded void * casts for the following (void *) pointers:
- struct file: private_data

The patch also contains some whitespace and coding style cleanups in the
relevant areas.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>