ata: pata_hpt366: check channel enable bits

HighPoint HPT36x chips did turn out to have the channel enable bits --
however, badly implemented. Make use of them, despite that is probably
only going to burden the driver's code -- assuming both channels are
always enabled by the HighPoint BIOS anyway...

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