b43: Fix warnings from gcc 4.6.0

gcc 4.6.0 warnings for b43:

  CC [M]  drivers/net/wireless/b43/lo.o
drivers/net/wireless/b43/lo.c: In function ‘lo_measure_gain_values’:
drivers/net/wireless/b43/lo.c:304:7: warning: variable ‘trsw_rx’ set but not used [-Wunused-but-set-variable]
  CC [M]  drivers/net/wireless/b43/dma.o
drivers/net/wireless/b43/dma.c: In function ‘free_all_descbuffers’:
drivers/net/wireless/b43/dma.c:760:30: warning: variable ‘desc’ set but not used [-Wunused-but-set-variable]
drivers/net/wireless/b43/dma.c: In function ‘b43_dma_handle_txstatus’:
drivers/net/wireless/b43/dma.c:1391:30: warning: variable ‘desc’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>