Input: tegra-kbc - correct call to input_free_device

If kzalloc for kbc fails, then we have NULL pointer dereference while
calling input_free_device(kbc->idev) in the error handling.
So it is safer to always use the original name, input_dev.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>