le_audio: Add missing current state of group check

This CL adds missing check of current group state while trying to stop
group.

Tag: #feature
Bug: 263686512
Test: atest --host bluetooth_le_audio_test bluetooth_le_audio_client_test
Merged-In: I8c9fc2bd23c2d2cca7f0bab67e564d1d13762190
Change-Id: I8c9fc2bd23c2d2cca7f0bab67e564d1d13762190
(cherry picked from commit 73fbbc59)