libata-sff: Don't scan disabled ports when checking for legacy mode.

libata-sff.c checks for legacy mode by testing if both primary and
secondary ports on a controller are in legacy mode and selects legacy
if either one is. However on some south bridge chips (e.g AMD
SB600/SB700) the secondary port is not wired, and when it is disabled
by setting the disable bit in the PCI header it appears as a fixed
legacy port.

Prevent incorrect detection by not testing ports that are marked as
'dummy'

tj: Addressed Sergei's review points.  Other style edits.

Signed-off-by: Darren Stevens <darren@stevens-zone.net>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>