pata_hpt3x2n: check channel enable bits

The driver's prereset() method still doesn't check the channel enable bits.
The bug was there for the entire time the driver has existed. :-/

Alan Cox fixed the HPT37x driver in commit b5bf24b9 ("[PATCH] hpt37x:
Check the enablebits") but forgot to check the HPT3x2N driver which has
the same bug. :-/

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