[media] em28xx: fix PCTV 290e LNA oops

Pointer to device state has been moved to different location during
some change. PCTV 290e LNA function still uses old pointer, carried
over FE priv, and it crash.

Reported-by: Janne Kujanpää <jikuja@iki.fi>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>