drivers: gnss: use absolute values for signed fractionals

Printing fractionals currently put the sign on integer values on the
fractional part, for example:

longitude : -6.-207483333

Run an extra abs to get rid of the sign there for latitude, longitude
and altitude, compute the sign separately so it works for numbers
between -1 and 0 as well.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>