ahci: Acer SA5-271 SSD Not Detected Fix

commit 8bfd174312629866efa535193d9e563768ff4307 upstream.

(Correction in this resend: fixed function name acer_sa5_271_workaround; fixed
 the always-true condition in the function; fixed description.)

On the Acer Switch Alpha 12 (model number: SA5-271), the internal SSD may not
get detected because the port_map and CAP.nr_ports combination causes the driver
to skip the port that is actually connected to the SSD. More specifically,
either all SATA ports are identified as DUMMY, or all ports get ``link down''
and never get up again.

This problem occurs occasionally. When this problem occurs, CAP may hold a
value of 0xC734FF00 or 0xC734FF01 and port_map may hold a value of 0x00 or 0x01.
When this problem does not occur, CAP holds a value of 0xC734FF02 and port_map
may hold a value of 0x07. Overriding the CAP value to 0xC734FF02 and port_map to
0x7 significantly reduces the occurrence of this problem.

Link: https://bugzilla.kernel.org/attachment.cgi?id=253091
Signed-off-by: Sui Chen <suichen6@gmail.com>
Tested-by: Damian Ivanov <damianatorrpm@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>