power_supply: 88pm860x_charger: Fix possible NULL pointer dereference and use of initialized variable

Do not put reference to power supply in early exit paths of
pm860x_done_handler() because:
1. it is not yet initialized,
2. it is NULL.

This fixes possible NULL pointer dereference and following build
warning:
drivers/power/88pm860x_charger.c: In function ‘pm860x_done_handler’:
drivers/power/88pm860x_charger.c:516:18: warning: ‘psy’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Additionally this puts the power supply reference before unlocking
mutex. This actually is not needed (there is no race here) but has
logical sense and makes the exit paths cleaner.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>