ide: fix comment typo in ide.h

Fix typo in the comment to the 'dma_mode' field of the 'struct ide_drive_s'
introduced by the commit 3fccaa19

 (ide: add
drive->dma_mode field).

Whilt at it, convert spaces to a tab in the declaration of the neighbouring
'dn' field...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>