cpumask: cpulist_parse() comments correction

As introduced in Rusty's commit 29c0177e

, the function has no
parameter @len, so need to remove it from comments to avoid kernel-doc
warning:

alexs@debian:~/linux-next$ scripts/kernel-doc -man
include/linux/cpumask.h | split-man.pl /tmp/man
....
Warning(include/linux/cpumask.h:602): Excess function parameter 'len'
description in 'cpulist_parse'

and correct the function name in comments to cpulist_parse.

Signed-off-by: Alex Shi <alex.shi@intel.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>