power: bq24257: Fix use of uninitialized pointer bq->charger

bq->charger is initialized in bq24257_power_supply_init.
Therefore, bq24257_power_supply_init should be called before the
registration of the IRQ handler bq24257_irq_handler_thread that calls
power_supply_changed(bq->charger).

Signed-off-by: Georges Savoundararadj <savoundg@gmail.com>
Cc: Aurelien Chanot <chanot.a@gmail.com>
Cc: Andreas Dannenberg <dannenberg@ti.com>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Fixes: 2219a935 ("power_supply: Add TI BQ24257 charger driver")
Signed-off-by: Sebastian Reichel <sre@kernel.org>