iio: core: Calculate alloc_size only once in iio_device_alloc()

No need to rewrite the value, instead use 'else' branch.
This will also help further refactoring the code later on.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240304140650.977784-3-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>