Feedback from review by Arjan @ Redhat:
o Cleanup: Removed unneccesary #ifdef/#endif wrappers for features 
  already part of kernel.
o Bug fix: Use kernel funcs pci_[clear|set]_mwi rather than using direct
  calls to pci_write_config_word.
o Bug fix: Added read-behind-write calls to post writes before delays.
o Bug fix: removed calls to mdelay in interrupt context.
o Clean up: removed calls to memset followed by mb() and replaced with
  direct clearing of descriptor bits.