Input: ads7846 - fix device usage within attribute show

With commit e585c40b (Input: ads7846 - convert to
hwmon_device_register_with_groups()) the device passed to the attribute's
show function isn't the spi device as before.
So fixup the passed device to ads7846_read12_ser.

Signed-off-by: Alexander Stein <alexanders83@web.de>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>