hpt366: add hpt3xx_disable_fast_irq() helper

* Factor out disabling of "fast interrupt" prediction from
  init_hwif_hpt366() to hpt3xx_disable_fast_irq() helper.

* Use hpt3xx_disable_fast_irq() in ->init_chipset instead of
  ->init_hwif method.

  This is a preparation for adding proper PCI PM support.

While at it:

* Cache chip type in chip_type variable in hpt3xx_disable_fast_irq().

There should be no functional changes caused by this patch.

Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>