media: usb: airspy: clean the freed pointer and counter

After urb was freed, the pointer and counter need to be
cleaned.

Signed-off-by: Nil Yi <teroincn@163.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>