[media] tlg2300: fix checking firmware in poseidon_probe()

check_firmware() makes sure firmware is in a device.
It returns zero on success and error code otherwise.
Also it sets down_firmware flag to 1 if downloading occurs.
The only caller poseidon_probe() checks down_firmware flag and
returns 0 without any initialization if it is set.
That looks very strange, so the patch removes down_firmware argument
of check_firmware() and returns error code if check_firmware() fails
in poseidon_probe().
Not tested on real hardware.
Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>