spl: Remove overwrite of relocated malloc limit

spl_init on some boards is called after stack and heap relocation, on
some platforms spl_relocate_stack_gd is called to handle setting the
limit to its value CONFIG_SPL_STACK_R_MALLOC_SIMPLE_LEN when simple
SPL malloc is enabled during relocation. spl_init should then not
re-assign the old pre-relocation limit when this is defined.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>