hpt366: use correct enablebits for HPT36x

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

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Acked-by: Linas Vepstas <linas@austin.ibm.com>
Cc: michal.kepien@poczta.onet.pl
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>