[netdrvr] tulip_read_eeprom fixes for BUG 4420

If "location" is > "addr_len" bits, the high bits of location would interfere
with the READ_CMD sent to the eeprom controller.

A patch was submitted to bug:
    http://bugzilla.kernel.org/show_bug.cgi?id=4420



which simply truncated the "location", read whatever was in "location
modulo addr_len", and returned that value. That avoids confusing the
eeprom but seems like the wrong solution to me.

Correct would be to not read beyond "1 << addr_len" address of the eeprom.
I am submitting two changes to implement this:
1) tulip_read_eeprom will return zero (since we can't return -EINVAL)
   if this is attempted (defensive programming).
2) In tulip_core.c, fix the tulip_read_eeprom caller so they don't
   iterate past addr_len bits and make sure the entire tp->eeprom[]
   array is cleared.

I konw we don't strictly need both. I would prefer both in the tree
since it documents the issue and provides a second "defense" from
the bug from creeping back in.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>