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>