power: supply: smb347-charger: Replace mutex with IRQ disable/enable

Let's simply disable/enable IRQ rather than use a mutex that protects from
racing with the interrupt handler. The result of this patch is that it's a
bit easier now to follow the driver's code.

Tested-by: David Heidelberg <david@ixit.cz>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>