iwlagn: use PCI_DMA_* for pci_* operations

"iwlagn: map command buffers BIDI" uses the DMA_* enumerations for DMA
directions, even though the pci_* DMA API is still in use.  That patch
was undoubtedly developed on top of "iwlagn: don't use the PCI wrappers
for DMA operation", which is due in the next release.

Signed-off-by: John W. Linville <linville@tuxdriver.com>