b43: fix build error if !CONFIG_B43_LEDS

Fix the following build error when CONFIG_B43_LEDS is not selected:

drivers/net/wireless/b43/main.c: In function 'b43_remove':
drivers/net/wireless/b43/main.c:4990: error: 'struct b43_leds' has no member named 'stop'
drivers/net/wireless/b43/main.c:4991: error: 'struct b43_leds' has no member named 'work'
make[4]: *** [drivers/net/wireless/b43/main.o] Error 1

Signed-off-by: Albert Herranz <albert_herranz@yahoo.es>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>