sata_nv: kill old private BMDMA helper functions

sata_nv implemented its own copies of the BMDMA helper functions for ADMA,
since the ADMA BMDMA status registers are PIO while the other registers
are MMIO, and this was the only way to handle this previously. Now that
we have iomap support, the standard routines should just work, so use them.
The only thing we need to override as far as ADMA and BMDMA is the
post_internal_cmd callback, where we should only call ata_post_internal_cmd
if we are in port-register mode.

Signed-off-by: Robert Hancock <hancockr@shaw.ca>
Signed-off-by: Jeff Garzik <jeff@garzik.org>