spi: rtio: null pointer dereference in spi_rtio_transceive

Check if cqe is NULL before accessing cqe->result in
spi_rtio_transceive(). Prevents possible null pointer dereference
from rtio_cqe_consume() return value.

CID: 516229
Fixes: #90547

Signed-off-by: sudarsan N <sudarsansamy2002@gmail.com>