V4L/DVB (7993): si470x: move global lock to device structure

this patch brings the following changes:
- move the global disconnect lock into the device structure
- code cleanup (spaces to tabs, long line splits, ...)

Signed-off-by: Tobias Lorenz <tobias.lorenz@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>