stand/efi/eficom: better handling of absent device

Don't even have a comc_port when the port doesn't exist: always free it
if we probe that it's not there. Also, when it's not present, clear the
flags indicating presence to avoid disturbing other flags.

Sponsored by: Netflix
Reviewed by: tsoome, kevans
Differential Revision: https://reviews.freebsd.org/D40220