bluetooth: leaudio: Termination of PA at Modify Source

From BASS spec:
If the PA_Sync parameter value written by the client is set to a value
of 0x00 (Do not synchronize to PA) and the server is synchronized to
the PA, the server shall stop synchronization with the PA and shall
write a value of 0x00 (Not synchronized to PA) to the PA_Sync_State
field of the Broadcast Receive State characteristic .

Fixes BASS/SR/CP/BV-12-C test case.

Signed-off-by: Magdalena Kasenberg <magdalena.kasenberg@codecoup.pl>