USB: serial: console: clean up sanity checks

Drop two redundant NULL checks from usb_serial_console_disconnect().

The usb_serial_console_disconnect function is called from the
USB-serial-device disconnect callback when a device is going away. Hence
there is no need to check for the serial-device pointer being NULL.

The serial-device port pointers are stored in an array that is a member
of the serial struct so the address of the first member of the array
(which the array name decays to) is never NULL either.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>