net: qualcomm: rmnet: rearrange some NOTs

With the ones' complement arithmetic, the sum of two negated values
is equal to the negation of the sum of the two original values [1].
Rearrange the calculation ip6_payload_sum using this property.

[1] https://tools.ietf.org/html/rfc1071

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>