PCI: Unify PCI and normal DMA direction definitions

Current DMA direction definitions in pci-dma-compat.h and dma-direction.h
are mirrored in value.  Unify them to enhance readability and avoid
possible inconsistency.

Signed-off-by: Shunyong Yang <shunyong.yang@hxt-semitech.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Joey Zheng <yu.zheng@hxt-semitech.com>