soc: apple: rtkit: Make shmem_destroy optional

shmem_destroy isn't always required for coprocessor-managed buffers but we
still enforce that it exists. Just relax the check.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Signed-off-by: Sven Peter <sven@kernel.org>
Link: https://lore.kernel.org/r/20250610-smc-6-15-v7-4-556cafd771d3@kernel.org


Signed-off-by: Lee Jones <lee@kernel.org>