V4L/DVB (4668): Cx88: rename mpeg capability flags from CX88_BOARD_FOO to CX88_MPEG_FOO

The flags for mpeg capabilities are sub-optimally named as
CX88_BOARD_DVB and CX88_BOARD_BLACKBIRD, which creates some confusion.
This patch renames the above to CX88_MPEG_DVB and CX88_MPEG_BLACKBIRD.

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