it821x: use ide_get_pair_dev() helper

Because presence of the peer device was not checked in
it821x_set_pio_mode() PIO0 mode was used for taskfile PIO
in single device configurations.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>