V4L/DVB (5496): Pluto2: fix incorrect TSCR register setting

The ADEF bits in the TSCR register have different meanings in read and
write mode. For this reason ADEF has to be reset on every
read-modify-write operation.
This patch introduces a special write function for this register, which
takes care of it.

Thanks to Holger Magnussen for pointing my nose at this problem.

Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>