V4L/DVB (3982): Cx88-blackbird: use encoder firmware filename defined in cx2341x.h

Since the filename of the cx23416 encoder firmware image
is defined in cx2341x.h, we don't need to explicitly define
it in cx88-blackbird.c anymore.
This changeset removes BLACKBIRD_FIRM_ENC_FILENAME,
replacing all occurrances with CX2341X_FIRM_ENC_FILENAME.

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