cfg80211: mark ieee80211_hdrlen const

This function analyses only its single, value-passed
argument, and has no side effects. Thus it can be
const, which makes mac80211 smaller, for example:

   text	   data	    bss	    dec	    hex	filename
 362518	  16720	    884	 380122	  5ccda	mac80211.ko (before)
 362358	  16720	    884	 379962	  5cc3a	mac80211.ko (after)

a 160 byte saving in text size, and an optimisation
because the function won't be called as often.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

include/net/cfg80211.h

@@ -1715,7 +1715,7 @@ unsigned int ieee80211_get_hdrlen_from_skb(const struct sk_buff *skb);
  * ieee80211_hdrlen - get header length in bytes from frame control
  * @fc: frame control field in little-endian format
  */
-unsigned int ieee80211_hdrlen(__le16 fc);
+unsigned int __attribute_const__ ieee80211_hdrlen(__le16 fc);
 
 /**
  * ieee80211_data_to_8023 - convert an 802.11 data frame to 802.3

net/wireless/util.c

@@ -221,7 +221,7 @@ const unsigned char bridge_tunnel_header[] __aligned(2) =
 	{ 0xaa, 0xaa, 0x03, 0x00, 0x00, 0xf8 };
 EXPORT_SYMBOL(bridge_tunnel_header);
 
-unsigned int ieee80211_hdrlen(__le16 fc)
+unsigned int __attribute_const__ ieee80211_hdrlen(__le16 fc)
 {
 	unsigned int hdrlen = 24;