YAFFS2: cmd_yaffs2.c - fix build warnings

Fix these:
cmd_yaffs2.c: In function 'do_ywr':
cmd_yaffs2.c:69: warning: format '%x' expects type 'unsigned int', but argument 2 has type 'ulong'
cmd_yaffs2.c:69: warning: format '%x' expects type 'unsigned int', but argument 3 has type 'ulong'

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