mtd: st_spi_fsm: use %*ph to print small buffer

Use %*ph format to print small buffer as hex string.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Antonio Cardace <anto.cardace@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>