USB: serial: garmin_gps: don't compile unused packet definitions

Don't compile the four unused packet definitions but keep them around
for documentation purposes.

This avoids the corresponding W=1 (-Wunused-const-variable) warning.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>