[PATCH] usbserial: Fixes wrong return values.

Some usbserial functions returns -EINVAL if the port doesn't exist or if
it's not opened. However, the right error code for such situations is
-ENODEV.

Signed-off-by: Luiz Fernando N. Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>