mfd: twl4030-madc: Use twl_i2c_read/write_u16 for 16 bit registers

Simplify reading and writing of 16 bit TWL registers in the
driver by using twl_i2c_read_u16 and twl_i2c_write_u16.

Signed-off-by: Sebastian Reichel <sre@debian.org>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Tested-by: Marek Belisko <marek@goldelico.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>