ahci: st: Utilise ata_platform_remove_one() call

ata_platform_remove_one() allows us to specify our own exit function
via platform data then goes off and removes ATA Host and Port in
preparation for device removal.

Suggested-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Tejun Heo <tj@kernel.org>