mtd: fix 2 compiler warnings

mtd.c:544:7: warning: format ‘%x’ expects argument of type ‘unsigned int’, but argument 3 has type ‘ssize_t’ [-Wformat]
mtd.c:602:1: warning: label ‘done’ defined but not used [-Wunused-label]

Signed-off-by: John Crispin <blogic@openwrt.org>

SVN-Revision: 43383