bluetooth: bap: Fix shift of requested_bis_sync

The BIS_Sync bitfiled received over the air in Add Source and Modify
Source operations uses BIT(0) for BIS Index 1. Shift it when storing
it to match bis_sync bitfield where BIS Index 1 is at BIT(1).

Signed-off-by: Magdalena Kasenberg <magdalena.kasenberg@codecoup.pl>