V4L/DVB (4769): Added support for a ASUSTEK P7131 Dual DVB-T variant

This card has no firmware eeprom. The old version still should not
need a firmware file due to an undocumented feature of the TDA10046.
The patch also includes Hermann Pitton's proposal for improved
antenna switch handling

Signed-off-by: Hartmut Hackmann <hartmut.hackmann@t-online.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>