Bluetooth: controller: Allow any valid ISO sync receiver BIG handle

The BIG handle for an ISO sync receiver is provided by the host. The
valid range is [0x00..0xEF], and this requires the controller to allow
non-consecutive handle numbering. This means that even if only one sync
set is supported, a BIG handle of ex. value 2 must be supported.

As the code uses indexing and range checking directly by handle, this
needs to be changed. Introducing the handle index, which sequnces the
handles such that index = 0 is always the "first handle", regardless
of value.

Signed-off-by: Morten Priess <mtpr@oticon.com>