Fix the following W=1 kernel build warning(s):

 net/nfc/digital_core.c:473: warning: expecting prototype for start_poll operation(). Prototype was for digital_start_poll() instead
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>