b43: correct warning for uninitialized variable 'macstat'

  CC [M]  drivers/net/wireless/b43/pio.o
drivers/net/wireless/b43/pio.c: In function ‘pio_rx_frame’:
drivers/net/wireless/b43/pio.c:614:6: warning: ‘macstat’ may be used uninitialized in this function

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