efi/libstub: Fix the efi_load_initrd function description

The soft_limit and hard_limit in the function efi_load_initrd describes
the preferred and max address of initrd loading location respectively.
However, the description wrongly describes it as the size of the
allocated memory.

Fix the function description.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>