[le audio] Fix broadcast code padding on assistant side

According to Core Spec. 5.3, Vol 3, Part C, Sec. 3.2.6.3
The code should be LSB first, with MSB padding bytes when
needed. The source has the fix, but assistant side is missing this.

Tag: #feature
Bug: 270005590
Test: atest BluetoothInstrumentationTests
Test: manual broadcast test with encrypted source
Change-Id: Id068335d05fe2fe3c9cf3990b5458ec97d99e55f