iio: refactor info mask and ext_info attribute creation.

Introduce an enum to specify whether the attribute is separate or
shared.

Factor out the bitmap handling for loop into a separate function.

Tidy up error handling and add a NULL assignment to squish a false
positive warning from GCC.

Change ext_info shared type from boolean to enum and update in all
drivers.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>

drivers/iio/dac/mcp4725.c

@@ -195,8 +195,9 @@ static const struct iio_chan_spec_ext_info mcp4725_ext_info[] = {
 		.name = "powerdown",
 		.read = mcp4725_read_powerdown,
 		.write = mcp4725_write_powerdown,
+		.shared = IIO_SEPARATE,
 	},
-	IIO_ENUM("powerdown_mode", false, &mcp4725_powerdown_mode_enum),
+	IIO_ENUM("powerdown_mode", IIO_SEPARATE, &mcp4725_powerdown_mode_enum),
 	IIO_ENUM_AVAILABLE("powerdown_mode", &mcp4725_powerdown_mode_enum),
 	{ },
 };