arch_init: Clean up the duplicate variable 'len' defining in ram_load()

There are two places that define 'len' variable, It's OK for compiling,
but makes it difficult for reading.

Remove the local one which defined in the inside 'while' loop.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
