power: charger_manager: convert to use match_string() helper

The new helper returns index of the mathing string in an array.  We
would use it here.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>