slipif: Fix unbalance SYS_ARCH_PROTECT in slipif_process_rxqueue()

Fixes: 3d1a306518a5 ("SLIP netif: add support for multiple input strategies (threaded, polling, RX from ISR)")
Signed-off-by: Axel Lin <axel.lin@ingics.com>

src/netif/slipif.c

@@ -471,6 +471,7 @@ slipif_process_rxqueue(struct netif *netif)
     }
     SYS_ARCH_PROTECT(old_level);
   }
+  SYS_ARCH_UNPROTECT(old_level);
 }
 
 /** Like slipif_rxbyte, but queues completed packets.