iio: ABI: Fix name of timestamp sysfs file

The name of the file is "current_timetamp_clock" not
"timestamp_clock".

Fixes: bc2b7dab

 ("iio:core: timestamping clock selection support")
Cc: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>