fs/yaffs2/yaffs_guts.c: Fix GCC 4.6 compile warning (and bug)

Fix:
yaffs_guts.c: In function 'yaffs_GarbageCollectBlock':
yaffs_guts.c:2761:6: warning: variable 'retVal' set but not used
[-Wunused-but-set-variable]

Here GCC actually detected a bug.  The code was always returning OK
instead of the previously set retrun code.  Fix that.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: William Juul <wiljuul@cisco.com>
Cc: Scott Wood <scottwood@freescale.com>
Acked-by: William Juul <wiljuul@cisco.com>