V4L/DVB (3337): Drivers/media/dvb/frontends/mt312.c: cleanups

This patch contains the following possible cleanups:
- update the Kconfig help to mention the VP310
- merge vp310_attach and mt312_attach into a new vp310_mt312_attach
  to remove some code duplication

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>