The AX88190 has 64k of external SRAM, of which 62k can be used for
packet data. However, the AX88190A moves this on-chip and reduces it
to the more traditional 16k from 16k-32k. The AX88790 follows the
'190A. Probe memory above 32k to see which flavor of the '190 we have
and use the extra memory if we have it.

Eliminate the kludgy read eeprom for the ID code. It really is just a
memory read at location 0x400, so just use that instead. Makes the
code easier to understand as well as eliminates some magic numbers.