USB: serial: clean up dtr_rts

Since commit 957dacae ("TTY: fix DTR not being dropped on hang up")
dtr_rts is no longer called for uninitialised ports (e.g. after
a disconnect), which removes the need to grab the disconnect mutex.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>