IIO: si7020: Allocate correct amount of memory in devm_iio_device_alloc

Since only a pointer to struct i2c_client is stored in a private area
of IIO device created by the driver there's no need to allocate
sizeof(struct i2c_client) worth of storage.

Pushed to stable as this is linked to the revert patch previously.
Without this followup the original patch looks sensible.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>