Tweak user-bit math

[x42]

Remaining part from #51 -- needs platform tests and check (ARM in particular)

diff --git a/src/ltc.c b/src/ltc.c
index bc54353..03afb1d 100644
--- a/src/ltc.c
+++ b/src/ltc.c
@@ -312,21 +312,21 @@ void ltc_encoder_set_user_bits(LTCEncoder *e, unsigned long data){

 unsigned long ltc_frame_get_user_bits(LTCFrame *f){
  unsigned long data = 0;
- data |= f->user8;
+ data += f->user8;
  data <<= 4;
- data |= f->user7;
+ data += f->user7;
  data <<= 4;
- data |= f->user6;
+ data += f->user6;
  data <<= 4;
- data |= f->user5;
+ data += f->user5;
  data <<= 4;
- data |= f->user4;
+ data += f->user4;
  data <<= 4;
- data |= f->user3;
+ data += f->user3;
  data <<= 4;
- data |= f->user2;
+ data += f->user2;
  data <<= 4;
- data |= f->user1;
+ data += f->user1;
  return data;
 }