libata: export ata_port port_no attribute via /sys

While registering host controller track port number based upon number
of ports available on the controller, export port_no attribute through
/sys. This patch is needed by udev for composing persistent links in
/dev/disk/by-path.

/sys/devices/pci0000:00/0000:00:1f.2/ata8/ata_port/ata8
total 0
lrwxrwxrwx. 1 root root    0 Mar  6 12:43 device -> ../../../ata8
-r--r--r--. 1 root root 4096 Mar  6 12:43 idle_irq
-r--r--r--. 1 root root 4096 Mar  6 12:43 nr_pmp_links
-r--r--r--. 1 root root 4096 Mar  6 12:43 port_no
drwxr-xr-x. 2 root root    0 Mar  6 12:42 power
lrwxrwxrwx. 1 root root    0 Mar  6 12:41 subsystem -> ../../../../../../class/ata_port
-rw-r--r--. 1 root root 4096 Mar  6 12:40 uevent
1

Signed-off-by: David Milburn <dmilburn@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>