[media] rtl28xxu: do not hard depend on staging SDR module

RTL2832 SDR extension module is currently on staging. SDR module
headers were included from staging causing direct dependency staging
directory. As a solution, add needed headers to main driver.
Motivation of that change comes from Luis / driver backports project.

Reported-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Cc: backports@vger.kernel.org
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>