YAFFS2: fs/yaffs2/yaffs_guts.c - fix build warnings

Fix these:
yaffs_guts.c: At top level:
yaffs_guts.c:400: warning: 'yaffs_SkipFullVerification' defined but not used

Testing shows no changes of the image sizes.

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