USB: keyspan_pda: remove bogus disconnect test from dtr_rts

Remove bogus (and unnecessary) test for serial->dev being NULL in
dtr_rts.

The device is never cleared, and disconnect is handled for dtr_rts in
usb-serial core anyway.

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