mfd: adp5520: Use module_i2c_driver()

module_i2c_driver() removes some boilerplate and makes the code
simple.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>