Bluetooth: ISO/BAP: Refactor BIS bitfield

Refactors teh BIS bitfield values used for ISO
and BAP.

Previously BIT(1) meant BIS index 1, which was a Zephyr choice
in the early days of ISO, as the BT Core spec did not use
a bitfield for BIS indexes.

Later the BASS specification came along and defined that
BIT(0) meant BIS index 1, which meant that we had to shift BIS
bitfields between BAP and ISO.

This commit refactors the ISO layer to use BIT(0) for Index 1 now,
which means that there is no longer a need for conversion
between the BAP and ISO layers, and that we can use a value
range defined by a BT Core spec (BASS).

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>