eepro100: Don't allow guests to fail assertions

The idea of using assert() for input validation is rather questionable.
Let's remove it from eepro100, so that guests need to find more interesting
ways if they want to crash qemu.

This patch replaces asserts that are directly dependent on guest-accessible
data by other means of error handling.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
