LeAudio: Fix broadcast code padding

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.

Tag: #feature
Bug: 266444992
Test: atest BluetoothInstrumentationTests
Test: verified in btsnoop logs
Change-Id: I1101138b727633825fa5ea253d7d2197e30769c5