cpumask: Fix typo nr_cpumask_size --> nr_cpumask_bits

The never used nr_cpumask_size is just a typo, hence use existing
redefinition that's called nr_cpumask_bits.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>