drivers/net/wan/pc300_drv: fix bug caught by gcc warning

The warning

drivers/net/wan/pc300_drv.c: In function ‘cpc_open’:
drivers/net/wan/pc300_drv.c:2942: warning: ‘br’ may be used
uninitialized in this function

was valid.  Ensure 'br' is initialized in all cases.

Signed-off-by: Jeff Garzik <jeff@garzik.org>