lp8727_charger: Use devm_kzalloc()

Use devm_kzalloc() rather than kzalloc()/kfree() to make
allocating/freeing the private data simpler.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>