bitmap: replace _reg_op(REG_OP_ALLOC) with bitmap_set()

_reg_op(REG_OP_ALLOC) duplicates bitmap_set().

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