Bluetooth: csis: Expose bt_csis_generate_rsi function

The function will be used to update the RSI value. The returned value
can be later used in advertisement data for set identification. This
removes the need of specific bt_csis_advertise function so that the user
can call the API to generate new RSI once needed (e.g. on RPA change).
This allows the user to manage the advertisement data to fit it's needs.

< HCI Command: LE Set Extended Advertising Enable (0x08|0x0039) plen 6
        Extended advertising: Disabled (0x00)
        Number of sets: 1 (0x01)
        Entry 0
          Handle: 0x00
          Duration: 0 ms (0x00)
          Max ext adv events: 0
> HCI Event: Command Complete (0x0e) plen 4
      LE Set Extended Advertising Enable (0x08|0x0039) ncmd 1
        Status: Success (0x00)
< HCI Command: LE Set Extended Advertising Data (0x08|0x0037) plen 33
        Handle: 0x00
        Operation: Complete extended advertising data (0x03)
        Fragment preference: Minimize fragmentation (0x01)
        Data length: 0x1d
        Flags: 0x06
          LE General Discoverable Mode
          BR/EDR Not Supported
        Unknown EIR field 0x2e: 16e61d64dc45
        Name (complete): audio test shell
> HCI Event: Command Complete (0x0e) plen 4
      LE Set Extended Advertising Data (0x08|0x0037) ncmd 1
	Status: Success (0x00)
< HCI Command: LE Set Advertising Set Random Address (0x08|0x0035) plen 7
        Advertising handle: 0x00
        Advertising random address: 4E:21:29:F8:94:93 (Resolvable)
> HCI Event: Command Complete (0x0e) plen 4
      LE Set Advertising Set Random Address (0x08|0x0035) ncmd 1
        Status: Success (0x00)
< HCI Command: LE Set Extended Advertising Enable (0x08|0x0039) plen 6
        Extended advertising: Enabled (0x01)
        Number of sets: 1 (0x01)
        Entry 0
          Handle: 0x00
          Duration: 0 ms (0x00)
          Max ext adv events: 0
> HCI Event: Command Complete (0x0e) plen 4
      LE Set Extended Advertising Enable (0x08|0x0039) ncmd 1
        Status: Success (0x00)

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>