UBIFS: Fix dereferencing type-punned pointer compiler warning

Fix compiler warning

In file included from ubifs.h:2137:0,
                 from ubifs.c:26:
misc.h: In function 'ubifs_idx_key':
misc.h:263:26: warning: dereferencing type-punned pointer will break strict-aliasing rules

seen with gcc version 4.5.1 (Sourcery G++ Lite 2010.09-50).

No functional change.

CC: Stefan Roese <sr@denx.de>
Signed-off-by: Dirk Behme <dirk.behme@googlemail.com>
Signed-off-by: Stefan Roese <sr@denx.de>