lp8727_charger: Clean up the interrupt handler

For better understanding, function name is changed. (lp8727_intr_config()
is replaced with lp8727_setup_irq().)

The private IRQ number is set when the IRQ is allocated successfully. This
data is used for releasing the IRQ on unloading the driver. Even the IRQ
number is not defined, the driver should be operated. In this case, just
return as 0.

In additional function lp8727_release_irq(), the workqueue is canceled and
the allocated IRQ is released.

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