nfc: hci: Introduce nfc_llc_del_engine() to reduce code duplication

Add a new helper to avoid code duplication between nfc_llc_exit() and
nfc_llc_unregister().

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>