mtd: oobtest: correct printf() format specifier for 'size_t'

Fixes warning:

   drivers/mtd/tests/oobtest.c: In function 'memcmpshow':
   drivers/mtd/tests/oobtest.c:129: warning: format '%x' expects type 'unsigned int', but argument 3 has type 'size_t'
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Roger Quadros <rogerq@ti.com>
Cc: Sekhar Nori <nsekhar@ti.com>

drivers/mtd/tests/oobtest.c

@@ -132,7 +132,7 @@ static size_t memcmpshow(loff_t addr, const void *cs, const void *ct, size_t cou
 	for (su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--, i++) {
 		res = *su1 ^ *su2;
 		if (res) {
-			pr_info("error @addr[0x%lx:0x%x] 0x%x -> 0x%x diff 0x%x\n",
+			pr_info("error @addr[0x%lx:0x%zx] 0x%x -> 0x%x diff 0x%x\n",
 				(unsigned long)addr, i, *su1, *su2, res);
 			bitflips += hweight8(res);
 		}