Input: ads7846 - fix uninitialized var warning

drivers/input/touchscreen/ads7846.c: In function 'ads7846_read12_ser':
drivers/input/touchscreen/ads7846.c:216: warning: 'sample' may be used uninitialized in this function

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>