smc91111_eeprom: fix linking error

Building for a bf533-stamp ends up with this error:
smc91111_eeprom.o: In function `smc91111_eeprom':
examples/standalone/smc91111_eeprom.c:58: undefined reference to `memset'
make[2]: *** [smc91111_eeprom] Error 1

The new eth_struct definition means gcc has to zero out the structure on
the stack, and some gcc versions optimize this with an implicit call to
memset.  So tweak the structure style to avoid that gcc feature.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>