libata-core: kill duplicate statement in ata_do_set_mode()

Commit b3a706014e56b1356e7b275fd25b833c63175bf0 (libata: Add a
drivers/ide style DMA disable) neglected to remove the line in
ata_do_set_mode() it has obviously made useless/duplicated.  Do this
now, and make a line added back then wrapped properly...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>