staging:iio:buffer stop allowing for read_last callback.

No longer needed now we don't allow sysfs acccess to buffer data.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>