6lowpan: nhc: move iphc manipulation out of nhc

This patch moves the iphc setting of next header commpression bit inside
iphc functionality. Setting of IPHC bits should be happen at iphc.c file
only.

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