libata: fix checking of DMA state

Checking if DMA is enabled should be done via the
ata_dma_enabled helper function, since the init state
0xff indicates disabled.
This meant that ATA_CMD_READ_LOG_DMA_EXT was used and probed
for before DMA was enabled, which caused hangs for some combinations
of controllers and devices.
It might also have caused it to be incorrectly disabled as broken,
but there have been no reports of that.

Cc: stable@vger.kernel.org
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=195895


Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Tested-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>