sys/luid: luid_custom() use fixed width int

This changes the type of the last parameter of `luid_custom()` to a
fixed width integer for consistent behavior among different
architectures.

sys/include/luid.h

@@ -180,7 +180,7 @@ void luid_netdev_get_eui64(const netdev_t *netdev, eui64_t *addr);
  * @param[in]  len      length of the LUID in bytes
  * @param[in]  gen      custom LUID generator value
  */
-void luid_custom(void *buf, size_t len, int gen);
+void luid_custom(void *buf, size_t len, uint16_t gen);
 
 /**
  * @brief   Get a LUID base value

sys/luid/luid.c

@@ -67,7 +67,7 @@ void luid_get_lb(void *buf, size_t len)
     ((uint8_t *)buf)[len - 1] ^= lastused++;
 }
 
-void luid_custom(void *buf, size_t len, int gen)
+void luid_custom(void *buf, size_t len, uint16_t gen)
 {
     luid_base(buf, len);