iio: accel: sca3000: memory corruption in sca3000_read_first_n_hw_rb()

"num_read" is in byte units but we are write u16s so we end up write
twice as much as intended.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>