libata/pdc_adma: make SFF EH handle non-bmdma SFF drivers and standardize pdc_adma ops

pdc_adma has interface similar to SFF but has its own DMA interface.
It currently implements noop bmdma ops to avoid crashing
ata_bmdma_error_handler() which BTW actually is EH for SFF drivers.

This patch makes ata_bmdma_error_handler() dereference bmdma ops iff
bmdma_addr is initialized as done in ata_bmdma_post_internal_cmd.
This change allows pdc_adma to standardize ops and use SFF
error_handler and post_internal_cmd.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Mark Lord <liml@rtr.ca>
Signed-off-by: Jeff Garzik <jeff@garzik.org>