media: rtl2832_sdr: clean the freed pointer and counter

After freed the dev->urb_list, we should set it to NULL as well as set
counter to zero.

Requested-by: Sean Young <sean@mess.org>
Signed-off-by: Nil Yi <teroincn@163.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>