FM Radio: avoid deadlock when disabling RDS

When RDS is being configured, the session mutex is hold. However,
when it is being disabled, RDS callbacks might still fire, and these
also try to hold the mutex, leading to a deadlock.

Temporary unlock the mutex while the vendor library is called to
work around.

Change-Id: Ic9751c7185fd8de4a7510225f54d23a22f042321