[media] rtl28xxu: silence error log about disabled rtl2832_sdr module

It printed a little bit too heavy looking error log "DVB: Unable to
find symbol rtl2832_sdr_attach()" when staging module was disabled.
Silence that error by introducing own version of dvb_attach() macro
without the error logging.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>