le_advertiser: Fix reduntant HCI Command

Make sure to disable periodic advertising only when it is needed to
avoid Comand Disallowed.

< HCI Command: LE Set Extended A.. (0x08|0x0039) plen 6  #18224 14:40:29.928161
        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              #18225 14:40:29.931694
      LE Set Extended Advertising Enable (0x08|0x0039) ncmd 1
        Status: Success (0x00)
< HCI Command: LE Set Periodic A.. (0x08|0x0040) plen 2  #18226 14:40:29.931988
        Periodic advertising: Disabled (0x00)
        Handle: 0
> HCI Event: Command Complete (0x0e) plen 4              #18227 14:40:29.938424
      LE Set Periodic Advertising Enable (0x08|0x0040) ncmd 1
        Status: Command Disallowed (0x0c)

Bug: 339689853
Test: atest bluetooth_test_gd_unit
Test: Manual test with nRF connect and local Broadcaster
Flag: Exempt, trivial fix, regression checked with unit tests
Change-Id: I24539459f2e7ed9cb451dea8b92c3637f1f3ba5e