iio:pressure:mpl3115: Force alignment of buffer

Whilst this is another case of the issue Lars reported with
an array of elements of smaller than 8 bytes being passed
to iio_push_to_buffers_with_timestamp(), the solution here is
a bit different from the other cases and relies on __aligned
working on the stack (true since 4.6?)

This one is unusual.  We have to do an explicit memset() each time
as we are reading 3 bytes into a potential 4 byte channel which
may sometimes be a 2 byte channel depending on what is enabled.
As such, moving the buffer to the heap in the iio_priv structure
doesn't save us much.  We can't use a nice explicit structure
on the stack either as the data channels have different storage
sizes and are all separately controlled.

Fixes: cc26ad455f ("iio: Add Freescale MPL3115A2 pressure / temperature sensor driver")
Reported-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: Peter Meerwald <pmeerw@pmeerw.net>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200920112742.170751-7-jic23@kernel.org

drivers/iio/pressure/mpl3115.c

@@ -144,7 +144,14 @@ static irqreturn_t mpl3115_trigger_handler(int irq, void *p)
 	struct iio_poll_func *pf = p;
 	struct iio_dev *indio_dev = pf->indio_dev;
 	struct mpl3115_data *data = iio_priv(indio_dev);
-	u8 buffer[16]; /* 32-bit channel + 16-bit channel + padding + ts */
+	/*
+	 * 32-bit channel + 16-bit channel + padding + ts
+	 * Note that it is possible for only one of the first 2
+	 * channels to be enabled. If that happens, the first element
+	 * of the buffer may be either 16 or 32-bits.  As such we cannot
+	 * use a simple structure definition to express this data layout.
+	 */
+	u8 buffer[16] __aligned(8);
 	int ret, pos = 0;
 
 	mutex_lock(&data->lock);