V4L/DVB (3494): Kconfig: select VIDEO_MSP3400 to build msp3400.ko

The msp3400 driver is currently only being built if
the bttv driver is selected. There are new drivers
that will be needing msp3400, so simply including
msp3400 in the Makefile is no longer appropriate.
This patch creates VIDEO_MSP3400, and alters VIDEO_BT848, VIDEO_PVRUSB2
and VIDEO_AUDIO_DECODER each to select VIDEO_MSP3400.

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