Bluetooth: ISO: Support multiple BIGs

This adds support for creating multiple BIGs. According to
spec, each BIG shall have an unique handle, and each BIG should
be associated with a different advertising handle. Otherwise,
the LE Create BIG command will fail, with error code
Command Disallowed (for reusing a BIG handle), or
Unknown Advertising Identifier (for reusing an advertising
handle).

The btmon snippet below shows an exercise for creating two BIGs
for the same controller, by opening two isotest instances with
the following command:
    tools/isotest -i hci0 -s 00:00:00:00:00:00

< HCI Command: LE Create Broadcast Isochronous Group (0x08|0x0068) plen 31
        Handle: 0x00
        Advertising Handle: 0x01
        Number of BIS: 1
        SDU Interval: 10000 us (0x002710)
        Maximum SDU size: 40
        Maximum Latency: 10 ms (0x000a)
        RTN: 0x02
        PHY: LE 2M (0x02)
        Packing: Sequential (0x00)
        Framing: Unframed (0x00)
        Encryption: 0x00
        Broadcast Code: 00000000000000000000000000000000

> HCI Event: Command Status (0x0f) plen 4
      LE Create Broadcast Isochronous Group (0x08|0x0068) ncmd 1
        Status: Success (0x00)

> HCI Event: LE Meta Event (0x3e) plen 21
      LE Broadcast Isochronous Group Complete (0x1b)
        Status: Success (0x00)
        Handle: 0x00
        BIG Synchronization Delay: 912 us (0x000390)
        Transport Latency: 912 us (0x000390)
        PHY: LE 2M (0x02)
        NSE: 3
        BN: 1
        PTO: 1
        IRC: 3
        Maximum PDU: 40
        ISO Interval: 10.00 msec (0x0008)
        Connection Handle #0: 10

< HCI Command: LE Create Broadcast Isochronous Group (0x08|0x0068)
        Handle: 0x01
        Advertising Handle: 0x02
        Number of BIS: 1
        SDU Interval: 10000 us (0x002710)
        Maximum SDU size: 40
        Maximum Latency: 10 ms (0x000a)
        RTN: 0x02
        PHY: LE 2M (0x02)
        Packing: Sequential (0x00)
        Framing: Unframed (0x00)
        Encryption: 0x00
        Broadcast Code: 00000000000000000000000000000000

> HCI Event: Command Status (0x0f) plen 4
      LE Create Broadcast Isochronous Group (0x08|0x0068) ncmd 1
        Status: Success (0x00)

Signed-off-by: Iulia Tanasescu <iulia.tanasescu@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>