ata: pata_pdc202xx_old: make static read-only array pio_timing const

The static array pio_timing is read-only so it make sense to make
it const.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>