media: em28xx: constify a new function

em28xx_duplicate_dev() is static. This were supposed to be
merged on the last patch, but somehow, I forgot "-a" when
I called git commit --amend.

Fixes: be7fd3c3 ("media: em28xx: Hauppauge DualHD second tuner functionality")
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>