net: lwm2m: ignore duplicate/older block transfers

During firmware download via block-wise transfer, we can see
packets occaionally get re-transmitted (normal logic in the
pending / retry functions).  However, both of these packets
end up coming through the reply handler and we should ignore
any block-wise transfer that has a current value less than
where we expect to be.

NOTE: This fixes K64F ethernet transfers where we were getting
too many packets back in the handler.

Signed-off-by: Michael Scott <michael.scott@linaro.org>