stm32/Makefile: Fix setting of define that enables WIZNET5K driver.

Commit 9670a15 missed one renaming of
MICROPY_PY_WIZNET5K to MICROPY_PY_NETWORK_WIZNET5K which prevented the
Wiznet interface from being enabled.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

ports/stm32/Makefile

@@ -459,7 +459,7 @@ endif
 ifneq ($(MICROPY_PY_NETWORK_WIZNET5K),0)
 WIZNET5K_DIR=drivers/wiznet5k
 INC += -I$(TOP)/$(WIZNET5K_DIR)
-CFLAGS_MOD += -DMICROPY_PY_WIZNET5K=$(MICROPY_PY_NETWORK_WIZNET5K) -D_WIZCHIP_=$(MICROPY_PY_NETWORK_WIZNET5K)
+CFLAGS_MOD += -DMICROPY_PY_NETWORK_WIZNET5K=$(MICROPY_PY_NETWORK_WIZNET5K) -D_WIZCHIP_=$(MICROPY_PY_NETWORK_WIZNET5K)
 ifeq ($(MICROPY_PY_LWIP),1)
 # When using MACRAW mode (with lwIP), maximum buffer space must be used for the raw socket
 CFLAGS_MOD += -DWIZCHIP_USE_MAX_BUFFER