ACPI / battery: get rid of negations in conditions

Simple conditions without negations inflict less cognitive load
on readers.

Rework conditional branches not to use negations. Also add braces
around single statement branches where their counterpart else-branches
consist of more than one statement as suggested in the paragraph 3 of
the coding style.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dmitry Rozhkov <dmitry.rozhkov@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>