nodemask: Drop duplicate check in for_each_node_mask()

The return value type is changed from 'int' to 'unsigned int' since
commit 0dfe5407 ("nodemask: Fix return values to be unsigned").
Besides, the conversion between 'int' and 'unsigned int' on the
parameter @node is guaranteed to be safe due to the limited range of
MAX_NUMNODES and CONFIG_NODES_SHIFT. By the way, '(node >= 0)' should
have been '(node) >= 0' actually.

It's unnecessary to check if their return values are greater or equal
to 0 in for_each_node_mask(). Remove it.

No functional change intended.

Signed-off-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>