watchdog: pcwd_usb: remove needless check before usb_free_coherent()

usb_free_coherent() is safe with NULL usb_pcwd->intr_buffer and
this check is not required.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200713055348.21620-1-vulab@iscas.ac.cn
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>