net: drivers: Slip can get the MTU set on it's interface

No need to store the mtu locally as the network interface already holds
it.

Change-Id: I4a1c3d164eefa28622dcb6cd510a66a825c3d9fb
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>
diff --git a/drivers/slip/slip.c b/drivers/slip/slip.c
index 41cc367..bbab996 100644
--- a/drivers/slip/slip.c
+++ b/drivers/slip/slip.c
@@ -62,7 +62,6 @@
 	uint16_t ll_reserve;	/* Reserve any space for link layer headers */
 	uint8_t mac_addr[6];
 	struct net_linkaddr ll_addr;
-	int mtu;
 
 #if defined(CONFIG_SLIP_STATISTICS)
 #define SLIP_STATS(statement)
@@ -173,7 +172,7 @@
 			}
 		}
 
-		if (slip->mtu > net_buf_headroom(frag)) {
+		if (net_if_get_mtu(iface) > net_buf_headroom(frag)) {
 			/* Do not add link layer header if the mtu is bigger
 			 * than fragment size. The first packet needs the
 			 * link layer header always.
@@ -383,10 +382,8 @@
 
 #if defined(CONFIG_SLIP_TAP)
 	slip->ll_reserve = sizeof(struct net_eth_hdr);
-	slip->mtu = 1500; /* assume for ethernet */
 #else
 	slip->ll_reserve = 0;
-	slip->mtu = 576; /* assume for tun */
 #endif
 	SYS_LOG_DBG("%sll reserve %d",
 #if defined(CONFIG_SLIP_TAP) && defined(CONFIG_NET_IPV4)
@@ -440,11 +437,13 @@
 #if defined(CONFIG_SLIP_TAP) && defined(CONFIG_NET_L2_ETHERNET)
 #define _SLIP_L2_LAYER ETHERNET_L2
 #define _SLIP_L2_CTX_TYPE NET_L2_GET_CTX_TYPE(ETHERNET_L2)
+#define _SLIP_MTU 1500
 #else
 #define _SLIP_L2_LAYER DUMMY_L2
 #define _SLIP_L2_CTX_TYPE NET_L2_GET_CTX_TYPE(DUMMY_L2)
+#define _SLIP_MTU 576
 #endif
 
 NET_DEVICE_INIT(slip, CONFIG_SLIP_DRV_NAME, slip_init, &slip_context_data,
 		NULL, CONFIG_KERNEL_INIT_PRIORITY_DEFAULT, &slip_if_api,
-		_SLIP_L2_LAYER, _SLIP_L2_CTX_TYPE, 127);
+		_SLIP_L2_LAYER, _SLIP_L2_CTX_TYPE, _SLIP_MTU);