b43legacy: fix "‘up_dev’ may be used uninitialized" warning

drivers/net/wireless/b43legacy/main.c: In function ‘b43legacy_op_dev_config’:
drivers/net/wireless/b43legacy/main.c:2468: warning: ‘up_dev’ may be used uninitialized in this function

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