USB: input: appletouch: fix up compiler warning

My last patch fixing up the dev_* messages caused a compiler warning
accidentally for an unused variable.  Fix this up, as it was my fault.

Cc: Alessandro Rubini <rubini@ipvvis.unipv.it>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>