Fix sandbox build on Ubuntu 10.04

Fix sandbox build on Ubuntu 10.04

gcc 4.4.3 (which is the default native compiler on x86-64 Ubuntu 10.04)
doesn't seem to like initializers for sub-fields of anonymous unions.
Solve this by replacing the initialization with an assignment. This
fixes:

lib/lz4_wrapper.c: In function ‘ulz4fn’:
lib/lz4_wrapper.c:97: error: unknown field ‘raw’ specified in initializer

Signed-off-by: Stephen Warren <[email protected]>
Reviewed-by: Tom Rini <[email protected]>
Acked-by: Simon Glass <[email protected]>