YAFFS2: fs/yaffs2/Makefile - fix build warnings

Drop the "-DNO_Y_INLINE" setting to fix these:
yaffs_guts.h:806: warning: 'yaffs_GetBlockInfo' defined but not used

Impact on image size is negligible - for the VCMA9 board the text
segment size grew from 496353 to 496357 bytes (i. e. 0.0008%);
total image size even remained constant.

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