airo: fix airo_get_encode{,ext} buffer overflow like I mean it...

"airo: airo_get_encode{,ext} potential buffer overflow" was actually a
no-op, due to an unrecognized type overflow in an assignment.  Oddly,
gcc only seems to tell me about it when using -Wextra...grrr...

Signed-off-by: John W. Linville <linville@tuxdriver.com>