The way it is currently used, there's no reason for 'uint' field to be
mutable, so constify it in order to allow 'aiodev' drivers to pass
sting literals in that field.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>