[media] Kconfig: rtl2832_sdr must depend on USB

Fixes error:
[next:master 7435/8702] ERROR: "usb_alloc_urb
[drivers/media/dvb-frontends/rtl2832_sdr.ko] undefined!

rtl2832_sdr driver implements own USB streaming for SDR data.
Logically that functionality belongs to USB interface driver, but
currently it is implemented here.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>