ide: add IDE_HFLAG_CLEAR_SIMPLEX host flag

* Rename 'simplex_stat' variable to 'dma_stat' in ide_get_or_set_dma_base().

* Factor out code for forcing host out of "simplex" mode from
  ide_get_or_set_dma_base() to ide_pci_clear_simplex() helper.

* Add IDE_HFLAG_CLEAR_SIMPLEX host flag and set it in alim15x3 (for M5229),
  amd74xx (for AMD 7409), cmd64x (for CMD643), generic (for Netcell) and
  serverworks (for CSB5) host drivers.

* Make ide_get_or_set_dma_base() test for IDE_HFLAG_CLEAR_SIMPLEX host flag
  instead of checking dev->device (BTW the code was buggy because it didn't
  check for dev->vendor, luckily none of these PCI Device IDs was used by
  some other vendor for PCI IDE controller).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>