ahci: avoid #ifdef DEBUG_AHCI bitrot

Debug code using #ifdef is susceptible to bitrot because the compiler
never checks the debug code.

This is easy to avoid, change the DPRINTF() macro to use if (DEBUG_AHCI)
and always give it a 0 or 1 value.

This also allows us to drop an #ifdef DEBUG_AHCI in ahci_start_dma()
since the compiler can now see the local variable is used.

The motivation for this change is a recent DEBUG_AHCI build failure due
to an outdated DPRINTF() format string.  From now on the compiler will
catch these errors.

Cc: John Snow <jsnow@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1415874281-7371-2-git-send-email-stefanha@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>