[PATCH] dvb: Fix integer overflow bug

Fix integer overflow bug in read_signal_strength() reported by Anthony
Leclerc.

Signed-off-by: Johannes Stezenbach <js@linuxtv.org>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>