Since if_gem is being touted as one of our more architecturally
correct network drivers with respect to busmaster DMA, go over it
with at duster to make other aspects of it a role model:

Eliminate the pci specific softc, it serves no rational purpose.

Use convenience resource allocation/deallocation functions to save
code and errorhandling.

Switch from bus_space_{read|write}_%u() to bus_{read|write}_%u()
functions and forget about tags and handles, the resource will know
about those, should they be needed. This also eliminates a number
of inconsistently named local variables.