PPP: Fix speling issues

Found by codespell

src/include/netif/ppp/pppoe.h

@@ -128,7 +128,7 @@ PACK_STRUCT_END
 #define PPPOE_TAG_RELAYSID    0x0110  /* relay session id */
 #define PPPOE_TAG_SNAME_ERR   0x0201  /* service name error */
 #define PPPOE_TAG_ACSYS_ERR   0x0202  /* AC system error */
-#define PPPOE_TAG_GENERIC_ERR 0x0203  /* gerneric error */
+#define PPPOE_TAG_GENERIC_ERR 0x0203  /* generic error */
 
 #define PPPOE_CODE_PADI       0x09    /* Active Discovery Initiation */
 #define PPPOE_CODE_PADO       0x07    /* Active Discovery Offer */

src/include/netif/ppp/vj.h

@@ -53,7 +53,7 @@ extern "C" {
  *
  * There are 5 numbers which can change (they are always inserted
  * in the following order): TCP urgent pointer, window,
- * acknowlegement, sequence number and IP ID.  (The urgent pointer
+ * acknowledgement, sequence number and IP ID.  (The urgent pointer
  * is different from the others in that its value is sent, not the
  * change in value.)  Since typical use of SLIP links is biased
  * toward small packets (see comments on MTU/MSS below), changes

src/netif/ppp/lcp.c

@@ -1499,7 +1499,7 @@ static int lcp_nakci(fsm *f, u_char *p, int len, int treat_as_reject) {
 	    break;
 	case CI_AUTHTYPE:
 		/* This is potentially dead code (#if !PPP_AUTH_SUPPORT)
-		 * Thus the double parantheses to mark the code explicitely
+		 * Thus the double parentheses to mark the code explicitly
 		 * disabled when building with clang
 		 */
 	    if ((0