gpsd: fix gpsd init script

Replace return with exit where appropriate.

Current init script uses return outside of functions resulting in
warnings such as the following when using bash as interpreter:

/etc/init.d/gpsd: line 98: return: can only `return' from a function or sourced script

Signed-off-by: Johan Hovold <johan.hovold@lundinova.se>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>