efi/libstub/arm: Fix spurious message that an initrd was loaded

Commit:

  ec93fc37 ("efi/libstub: Add support for loading the initrd from a device path")

added a diagnostic print to the ARM version of the EFI stub that
reports whether an initrd has been loaded that was passed
via the command line using initrd=.

However, it failed to take into account that, for historical reasons,
the file loading routines return EFI_SUCCESS when no file was found,
and the only way to decide whether a file was loaded is to inspect
the 'size' argument that is passed by reference. So let's inspect
this returned size, to prevent the print from being emitted even if
no initrd was loaded at all.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-efi@vger.kernel.org