lp8727_charger: Use specific definition

Add new LP8727_ICHG_SHIFT definition and replace a magic number. Reuse
definition for the size of interrupt register buffer.

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