iio: accel: mma9551_core: use size in words for word buffers

Change the prototype for the mma9551_read/write_*_words functions
to receive the length of the buffer in words (instead of bytes) since
we are using a word buffer. This will prevent users from sending an
odd number of bytes for a word array.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>