PPC MPC83xx: Fix MPC8323ERDB build warning

Fix:

mpc8323erdb.c: In function 'mac_read_from_eeprom':
mpc8323erdb.c:198:3: warning: dereferencing type-punned pointer will
break strict-aliasing rules [-Wstrict-aliasing]

Signed-off-by: Wolfgang Denk <wd@denx.de>
cc: Timur Tabi <timur@tabi.org>
cc: Kim Phillips <kim.phillips@freescale.com>