mips: kexec-elf-mips: fix not free in elf_mips_load()

In the function elf_mips_load(), crash_cmdline was alloced memory.
But it seems to forget to free it when last used at line 131.

Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Simon Horman <horms@verge.net.au>