qcacmn: use qdf_nbuf_headlen API to get the length in TX path

currently qdf_nbuf_len is being used to get the length of
the tx pkt, but this API also interprets the nbuf CB field
to adjust the length if extra_frags are present.
The concept of extra_frags is applicable only for legacy
drivers but in lithium as cb is not reset in the driver the
qdf_nbuf_len API is mis-calculating the length if extra_frag
field in cb is set by any network stack protocol.
To avoid this qdf_nbuf_headlen API is used to get the length
of the TX pkt.

Change-Id: Ie1e0b4b2168daf93ae77f4c995f5c3476a27b433

dp/wifi3.0/dp_tx.h

@@ -27,9 +27,13 @@
 
 #define DP_TX_MAX_NUM_FRAGS 6
 
-#define DP_TX_DESC_FLAG_SIMPLE		0x1
+/*
+ * DP_TX_DESC_FLAG_FRAG flags should always be defined to 0x1
+ * please do not change this flag's definition
+ */
+#define DP_TX_DESC_FLAG_FRAG		0x1
 #define DP_TX_DESC_FLAG_TO_FW		0x2
-#define DP_TX_DESC_FLAG_FRAG		0x4
+#define DP_TX_DESC_FLAG_SIMPLE		0x4
 #define DP_TX_DESC_FLAG_RAW		0x8
 #define DP_TX_DESC_FLAG_MESH		0x10
 #define DP_TX_DESC_FLAG_QUEUED_TX	0x20