Fix probing of dual channel 7899 with some fixes from current FreeBSD code, plus an extra ahc_flush_device_writes().
Fix probing of dual channel 7899 with some fixes from current FreeBSD
code, plus an extra ahc_flush_device_writes().

ok deraadt@ tdeval@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars