stm32mp: bsec: add check on null size in misc ops

Add a protection in misc bsec ops for request with null size.

For example OP-TEE error occurs when get_eth_nb() return 0 in
setup_mac_address() for unknown part number because U-Boot read 0 OTPs.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>