[media] cx25840: fix return logic when media entity init fails

There's no need to free state, as it was allocated via devm_kzalloc().

Also, let's return the error code, instead of something else.

Reported-by: Prabhakar Lad <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>