Rename the ioctl asic register and slot macros from ASIC_<xxx> to
IOASIC_<xxx>, to be compatible with the machine-indpendent names in
sys/dev/tc/ioasicvar.h. The pmax code still uses
sys/arch/pmax/pmax/asic.h, as some of the registers and offsets
defined there are not yet defined in sys/dev/tc/ioasicvar.h.
Rename the ioctl asic base-address pointer from `asic_base' to `ioasic_base'.