USB: serial: garmin_gps: annotate struct garmin_packet with __counted_by

Use the __counted_by compiler attribute for the data[] flexible array
member to improve the results of array bound sanitizers.

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>