sockets.c: Make select() not return errors when using it with file descriptor types...

sockets.c: Make select() not return errors when using it with file descriptor types other than sockets

lwip's select() implementation supports only sockets. Up until now, we returned
errors when using select() with other file descriptor types. These changes make
it possible to use other file descriptor types as well, even though they will
be ignored.

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Stefan Teodorescu <stefanl.teodorescu@gmail.com>