Parentheses are missing around the macro argument, causing the
macro possibly not to work passing certain expressions as
arguments.

This should not cause any issues with current code, however it's
worth to add them, as a good practice, and to eventually avoid
future bugs.
Suggested-by: NDave Kilroy <kilroyd@googlemail.com>
Signed-off-by: NAndrea Merello <andrea.merello@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>