Input: ad7879 - use i2c_smbus_read_i2c_block_data() to lower overhead

Avoid additional addressing overhead incurred by word_data transfers.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>