samples: Bluetooth: central_iso: Remove K_FOREVER for buf alloc

Allocating a buffer in the system workqueue thread should never
be done wiht a K_FOREVER as that may block the thread forever.

Replace the K_FOREVER with a K_NO_WAIT and slightly modified
the function to handle this case.

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