While the code is correct, less sophisticated static analyzers (and
users operating them..) trip over it, because they don't see that
the be32_to_cpu argument is evaluated multiple times only if it's
constant. Moving the side effect out:

- Lets us avoid the false positive
- Aligns us with what Linux does
- Makes the code IMO a bit clearer

Do so.

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>