Bluetooth: Audio: Fix issue with registering VOCS callbacks

When registering VOCS callbacks via bt_vcs_client_cb_register
it could fail if the application did not reset the
vocs_cb.discover as that is set internally.

This commit adds a simple check to also allow the
vocs_cb.discover to be the callback VCS expect/want it to be.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>