net: 6lo: Fix typo in compress bit calculation

commit 2e3e93dc ("net: 6lo: Fix compress bit calculation") adds
a bit shift to the compress bit calculation, however it's typo'd as
a greater than comparison causing get_6co_compress() to return a
boolean value rather than the shifted version of the calculation.

Change-Id: Idacac2c032a03f1f75a78c94bed4c70428b8c77a
Signed-off-by: Michael Scott <michael.scott@linaro.org>