atkbdc: use proper pointer type for KBDC

Proper locking for atkbdc will likely replace the kbdc_lock mechanism
entirely with a mutex in atkbdc_softc, so that other consumers can also
properly ensure locking protocol is followed (e.g. psm.c:doinitialize).

The first step to doing this neatly is making KBDC less opaque so that
others don't have to jump through weird casting hoops to address the mutex.

No functional change intended; this diff effectively just removes a bunch of
casting. A future change may remove the KBDC typedef entirely and just opt
for using atkbdc_softc_c * directly, but this was decidedly a good
intermediate step to make these changes simple to audit.