Revert "rtw88: split driver up into a core and pci part"

After some discussion, for now, simply revert the change to split
the driver up into if_rtw88_core.ko and if_rtw88_pci.ko as we do
not have an if_rtw88.ko anymore. We do have code trying to
auto-load modules, e.g. ifconfig, based on if_xxx.ko.

We could, based on Makefile magic or further code, generate a
if_rtw88.ko module with proper dependencies and keep this but for
simplicity stay with the one slightly larger module for now.
Should code appear to do this "properly" we can revisit this once
USB support has landed.

Slightly update the module Makefile to keep the separation of files
between core and pci bits visible and maintainable for the future.

This reverts commit 0f7b9777f8f39fbc230b3e1de2f844d9f839adea.