V4L/DVB (5734): Cx88: kill dev->fw_size

Now that cx88-blackbird is only accepting the official firmware image,
we no longer have any need to store the size of the firmware inside the
cx88 data structure.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>