ata: pata_atiixp: make static read-only arrays const

The static arrays pio_timings and mwdma_timings are read-only so
it make sense to make them 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>