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>