ath(4): Remove MIPS AHB frontend and join PCI one w/ main support again

Following the removal of general MIPS support, there's no longer a need
to have the AHB bus-frontend in place, which according to Linux sources
also isn't used with any non-MIPS SoCs. For simplicity, PCI bus support
is only made conditional on the main one again, i. e. device ath_pci is
removed, and built into the main module, i. e. if_ath_pci.ko obsoleted,
respectively.
Effectively, this reverts the following commits and associated changes:
dba9c8597747c6c9bf3d2ec68f7eb90552878dc7
e849bb3ecbb1963344a22ae77fc96f89fbebf40c

Approved by: adrian
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D41354