watchdog: pcwd_usb: fix compilation warning

In function ‘usb_pcwd_probe’:
drivers/watchdog/pcwd_usb.c:611:12: warning: variable ‘maxp’ set but not
used [-Wunused-but-set-variable]
  int pipe, maxp;

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>