enc28j60: Prevent enc28j60_read() from running before init

This is a precaution in case enc28j60-ip64-driver.c is skipped.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>

dev/enc28j60/enc28j60.c

@@ -589,6 +589,10 @@ enc28j60_read(uint8_t *buffer, uint16_t bufsize)
   uint8_t status[2];
   uint8_t length[2];
 
+  if(!initialized) {
+    return -1;
+  }
+
   err = 0;
 
   setregbank(EPKTCNT_BANK);