ata: pata_artop: use *switch* in artop_init_one()

This driver uses a string of the *if* statements in artop_init_one()
where the *switch* statement would fit better.  While fixing this,
refactor the 6280 code to e.g. avoid a compound statement inside
the *case* section...

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>