rt2x00: Use __builtin_choose_expr() instead of ?:

To really force the FIELD macros to determine
the first bit of the register field we should
use the __builtin_choose_expr() function.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>