lib: crypto: fix comments for count_leading_zeros()

lib: crypto: fix comments for count_leading_zeros()

count_leading_zeros() is based on fls(), which is defined for x == 0,
contrary to __ffs() family. The comment in crypto/mpi erroneously
states that the function may return undef in such case.

Fix the comment together with the outdated function signature, and now
that COUNT_LEADING_ZEROS_0 is not referenced in the codebase, get rid of
it too.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Yury Norov <ynorov@nvidia.com>