MIPS: ralink: mt7621: use bitwise NOT instead of logical

It was the intention to reverse the bits, not make them all zero by
using logical NOT operator.

Fixes: cc19db8b ("MIPS: ralink: mt7621: do memory detection on KSEG1")
Suggested-by: Chuanhong Guo <gch981213@gmail.com>
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>