isdn: isdn_tty: fix build warning of strncpy

Not upstream as isdn is long deleted.

Fix up a strncpy build warning for isdn_tty_suspend() using strscpy.

It's not like anyone uses this code anyway, and this gets rid of a build
warnings so that we can see real warnings as they pop up over time.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>