x86, cleanup: fix description of __fls(): __fls(0) is undefined

Ricardo M. Correia spotted that the use of __fls() in fls64() did
not seem to make sense. In fact fls64()'s implementation is fine,
but the description of __fls() was wrong. Fix that.

Reported-by: "Ricardo M. Correia" <Ricardo.M.Correia@Sun.COM>
Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>