remove unnecessary code in ata_piix

We set sata_curr_device to 0 right after returning from init_sata(), so there's
no point in setting it to the last scanned driver at this point.
Note: there are more duplicities with cmd_sata, but those might be required,
as the code seems to reset the entire controller on every scan, ignoring the
requested port number.

Signed-off-by: Pavel Herrmann <morpheus.ibis@gmail.com>