only SAMP_FREQ is writable

Will lead to SAMP_FREQ being written by any attempt to write
to the other exported attributes and hence a rather unexpected
result!
Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>