bitmap: align __reg_op() wrappers with modern coding style

Fix comments so that scripts/kernel-doc doesn't warn, and fix for-loop
stype in bitmap_find_free_region().

CC: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>