USB: fix use after free in wacom_sys.c

the following commit added a use after free
http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=3D3bea733ab21247290bd552dd6a2cd3049af9adef


Found by coverity (cid #1441)

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: "Ping Cheng" <pingc@wacom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>