arm64/efi: uefi_init error handling fix

There's one early memmap leak in uefi_init error path, fix it and
slightly tune the error handling code.

Signed-off-by: Dave Young <dyoung@redhat.com>
Acked-by: Mark Salter <msalter@redhat.com>
Reported-by: Will Deacon <will.deacon@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>