AHCI: use ATA_BUSY

ahci_hardreset() and ahci_p5wdh_hardreset() use bare numbers for the
BSY bit of the ATA status register, despite it's #define'd in
<linux/ata.h>.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tejun Heo <tj@kernel.org>