Revert "wireguard: use DEV_STATS_INC()"

This reverts commit 9942c194 which is
commit 93da8d75 upstream.

DEV_STATS_INC() is not in the tree as it breaks the Android abi, so
revert this commit as it breaks the build.

Bug: 161946584
Change-Id: Ifb53ff1c335048ef74c2fc26480a3689448f0808
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>