sata_via: Apply WD workaround only when needed on VT6421

Currently, workaround for broken WD drives is applied always, slowing
down all drives. And it has a bug - it's not applied after resume.

Apply the workaround only if the error really appears
(SErr == 0x1000500). This allows unaffected drives to run at full speed
(provided that no affected drive is connected to the controller).
Also make sure the workaround is re-applied on resume.

Tested on VT6421.
As SCR registers access is known to cause problems on VT6420 (and I
don't have it to test), keep the workaround applied always on VT6420.

Unaffected drive (Hitachi HDS721680PLA380):
Before:
$ hdparm -t --direct /dev/sdb
/dev/sdb:
 Timing O_DIRECT disk reads: 160 MB in  3.01 seconds =  53.16 MB/sec

After:
$ hdparm -t --direct /dev/sdb
/dev/sdb:
 Timing O_DIRECT disk reads: 200 MB in  3.01 seconds =  66.47 MB/sec

Affected drive (WDC WD5003ABYX-18WERA0):
Before:
$ hdparm -t --direct /dev/sda

/dev/sda:
 Timing O_DIRECT disk reads: 180 MB in  3.02 seconds =  59.51 MB/sec

After:
$ hdparm -t --direct /dev/sdb
/dev/sdb:
 Timing O_DIRECT disk reads: 156 MB in  3.03 seconds =  51.48 MB/sec
$ hdparm -t --direct /dev/sdb
/dev/sdb:
 Timing O_DIRECT disk reads: 180 MB in  3.02 seconds =  59.64 MB/sec

The first hdparm is slower because of the error:
[   50.408042] ata5: Incompatible drive: enabling workaround. This slows down transfer rate to ~60 MB/s
[   50.728052] ata5: SATA link up 1.5 Gbps (SStatus 113 SControl 310)
[   50.744834] ata5.00: configured for UDMA/133

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Tejun Heo <tj@kernel.org>