iio: adc: ti-ads1015: Add static assert to test if shifted realbits fit into storagebits

Add compile-time static_assert wrapper to verify that shifted realbits
fit into storagebits. The macro is implemented in a more generic way so
it can be used to verify other values if required.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Cc: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/20220328194725.149150-7-marex@denx.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>