nrfmin: fix type for NETOPT_PROTO

The type [is documented as `gnrc_nettype_t`][1] so it should by checked
as such.

[1]: https://github.com/RIOT-OS/RIOT/blob/f9a3bdf1a749f0e6a0d9e79eee652d8b31d4d9e0/sys/include/net/netopt.h#L210-L212

cpu/nrf5x_common/radio/nrfmin/nrfmin.c

@@ -490,9 +490,9 @@ static int nrfmin_get(netdev_t *dev, netopt_t opt, void *val, size_t max_len)
             return sizeof(uint16_t);
 #ifdef MODULE_GNRC_SIXLOWPAN
         case NETOPT_PROTO:
-            assert(max_len >= sizeof(uint16_t));
-            *((uint16_t *)val) = GNRC_NETTYPE_SIXLOWPAN;
-            return sizeof(uint16_t);
+            assert(max_len == sizeof(gnrc_nettype_t));
+            *((gnrc_nettype_t *)val) = GNRC_NETTYPE_SIXLOWPAN;
+            return sizeof(gnrc_nettype_t);
 #endif
         case NETOPT_DEVICE_TYPE:
             assert(max_len >= sizeof(uint16_t));