rt2x00: Fix HW crypto key handling

When a crypto key is being removed, rt2x00mac should not
reset the key->hw_key_idx value because that will prevent
the driver from removing the correct key from the hardware.

Furthermore ffz() starts counting at 0 instead of 1, so we don't
need to substract 1 from the resulting value.

Signed-off-by: Stephen Blackheath <tramp.enshrine.stephen@blacksapphire.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>