x86: check function status in EDD boot code

Without checking the return value of get_edd_info() and adding the
entry only in the success case, 6 devices show up under
/sys/firmware/edd/, no matter how many devices are actually present.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>