6lowpan: iphc: rename hc06_ptr pointer to hc_ptr

The hc06_ptr pointer variable stands for header compression draft-06. We
are mostly rfc complaint. This patch rename the variable to normal hc_ptr.

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