bitmap: drop _reg_op() function

Now that all _reg_op() users are switched to alternative functions,
_reg_op() machinery is not needed anymore.

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