wireless: restrict to 32 legacy rates

Since the standards only define 12 legacy rates, 32 is certainly
a sane upper limit and we don't need to use u64 everywhere. Add
sanity checking that no more than 32 rates are registered and
change the variables to u32 throughout.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>