IIO: si7020: Allocate correct amount of memory in devm_iio_device_alloc

IIO: si7020: Allocate correct amount of memory in devm_iio_device_alloc

commit e01becbad300712a28f29b666e685536f45e83bc upstream.

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>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>