wl12xx: silence tx_attr uninitialized warning in wl1271_tx_fill_hdr

  CC [M]  drivers/net/wireless/wl12xx/tx.o
drivers/net/wireless/wl12xx/tx.c: In function ‘wl1271_tx_fill_hdr’:
drivers/net/wireless/wl12xx/tx.c:288:6: warning: ‘tx_attr’ may be used uninitialized in this function

Signed-off-by: John W. Linville <linville@tuxdriver.com>