fix most sign and variable initialization warnings

Change-Id: I56e08db791152a9c95fc0d16bc75b702e403274d
diff --git a/miscutils/nandwrite.c b/miscutils/nandwrite.c
index 2ba6e3f..5f8d033 100644
--- a/miscutils/nandwrite.c
+++ b/miscutils/nandwrite.c
@@ -199,7 +199,7 @@
 			 */
 			break;
 		}
-		if (cnt < meminfo_writesize) {
+		if (cnt < (ssize_t) meminfo_writesize) {
 			if (IS_NANDDUMP)
 				bb_error_msg_and_die("short read");
 			if (!(opts & OPT_p))
@@ -218,7 +218,7 @@
 		}
 
 		mtdoffset += meminfo_writesize;
-		if (cnt < meminfo_writesize)
+		if (cnt < (ssize_t) meminfo_writesize)
 			break;
 	}