power: qpnp-charger: avoid unbalanced irq enable

Use disabled flag to track wether or not an interrupt
has been disabled. Only if it is currently disabled
allow the interrupt to be enabled again to avoid
unbalanced irq warnings.

Change-Id: I8a18e5d03eb11cb07354fef5613422a3c8b49567
Signed-off-by: David Keitel <dkeitel@codeaurora.org>