LCSAP: fix dissection of negative longitude and latitude

Bug: 10767
Change-Id: I4ab0ffd3bd22f1884895f3118673b433d980de2f
Reviewed-on: https://code.wireshark.org/review/5704
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>