Bluetooth: Mesh: Explicitly ignore gatt_service_register return code

The error code from gatt_service_register is being ignored to ensure
that repeated actions still complete. Do this explicitly by adding
(void) casts, resolving coverity warnings on accidental error code
ignores.

Fixes #33051.
Fixes #33074.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>