board/w7o/vpd.c: Fix GCC 4.6 compile warnings

Fix:
vpd.c: In function 'vpd_is_valid':
vpd.c:168:2: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
vpd.c:169:2: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
vpd.c:178:2: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
vpd.c: In function 'vpd_get_data':
vpd.c:281:6: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
vpd.c:291:6: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
vpd.c:297:6: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
vpd.c:302:6: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]

Signed-off-by: Wolfgang Denk <wd@denx.de>