lan9118: fix a buffer overflow

Fix a buffer overflow, reported by cppcheck:
[/src/qemu/hw/lan9118.c:849]: (error) Buffer access out-of-bounds: s.eeprom

All eeprom handling code assumes that the size of eeprom is 128,
except lan9118_eeprom_cmd. Fix this by restricting the address passed.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
