Fix bug - Implement bfin ata interrupt handler to avoid "irq 68 nobody cared" (v2)

Return IRQ_HANDLED when bfin ata device is busy.
http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3513

v1-v2:
 - fold api breakage fixing patch together.
 - mark 'static', not 'inline'.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>