gps.c: Adjust $GPsts parser to what I observe with GPS03

The behavior I observe is as follows:

* $PERC,GPsts,0,0,0... on cold start
* $PERC,GPsts,1,0,0... once it has a fix
* $PERC,GPsts,2,0,0... if fix persists for long time
* $PERC,GPsts,3,0,0... if antenna disconnects after fix
* $PERC,GPsts,2,0,0... if antenna is re-connected

Change-Id: Id8f4c0d9a91edb52dace9b64f3b3d97443256eee
1 file changed