USB: serial: ftdi_sio: drop GPIO line checking dead code

Now that gpiolib can track the validity of GPIO pins, there is no need
to check whether the line is valid in request().

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20201204164739.781812-5-maz@kernel.org
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[johan: amend commit message]
Signed-off-by: Johan Hovold <johan@kernel.org>