NTB spad entry item size is sizeof(u32), replace hardcoded 4 with it.

Link: https://lore.kernel.org/r/20221102141014.1025893-6-Frank.Li@nxp.comSigned-off-by: NFrank Li <frank.li@nxp.com>
Signed-off-by: NLorenzo Pieralisi <lpieralisi@kernel.org>
Acked-by: NManivannan Sadhasivam <mani@kernel.org>