Input: uinput - fix setting up device name

The check for non-empty device name was botched since we tried to account
for extra space for the terminating zero at the same time. Convert to
kstrndup() to avoid this problem.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Aristeu Rozanski <aris@ruivo.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>