libata: Fix ata_busy_wait() kernel docs

> Looks like you should use ata_busy_wait() here, rather than reproducing
> the same code again.

It waits in 10uS chunks while 1uS chunks were used in the workaround.
Could indeed do that once I know the fix is right. While I'm at it the
ata_busy_wait kerneldoc is borked so here's a fix

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>