6lowpan: iphc: cleanup use of lowpan_push_hc_data

This patch uses the lowpan_push_hc_data functions in several places
where we can use it. The lowpan_push_hc_data was introduced in some
previous patches.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>