sis5513: fix PIO setup for ATAPI devices

Clear prefetch setting before potentially (re-)enabling it in
config_drive_art_rwp() so the transition of the device type on
the port from ATA to ATAPI (i.e. during warm-plug operation)
is handled correctly.

This is a really old bug (it probably goes back to very early
days of the driver) but it was only affecting warm-plug operation
until the recent "ide: try to use PIO Mode 0 during probe if
possible" change (commit 60293364

).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Tested-by: David Fries <david@fries.net>
Signed-off-by: David S. Miller <davem@davemloft.net>