hw/lan9118.c: Add missing 'break' to fix buffer overrun

Add a missing 'break' statement to fix a buffer overrun when
executing the EEPROM write-all command. Spotted by Coverity
(see bug 887883).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
