V4L/DVB (4770): Fix mode switch of Compro Videomate T300

The board did not return to analog mode since the board specific
"demod sleep" function was not called. 

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