efi: fix 32bit kernel boot failed problem using efi

Commit 35d5134b
("x86/efi: Correct EFI boot stub use of code32_start")
imported a bug, which will cause 32bit kernel boot failed
using efi method. It should use the label's address instead
of the value stored in the label to caculate the address of
code32_start.

Signed-off-by: Fupan Li <fupan.li@windriver.com>
Reviewed-by: Matt Fleming <matt.fleming@intel.com>