staging: rtl8192u: fix sparse warning, make a function static

Fix the following sparse warning:
drivers/staging/rtl8192u/ieee80211/ieee80211_tx.c:240:22: warning: symbol 'ieee80211_alloc_txb' was not declared. Should it be static?
Signed-off-by: Teodora Baluta <teobaluta@gmail.com>
Reviewed-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8192u/ieee80211/ieee80211_tx.c

@@ -237,7 +237,7 @@ void ieee80211_txb_free(struct ieee80211_txb *txb) {
 	kfree(txb);
 }
 
-struct ieee80211_txb *ieee80211_alloc_txb(int nr_frags, int txb_size,
+static struct ieee80211_txb *ieee80211_alloc_txb(int nr_frags, int txb_size,
 						 gfp_t gfp_mask)
 {
 	struct ieee80211_txb *txb;