Staging: gdm724x: mux: Check return value of register_lte_tty_driver().

Check the return value of of the register_lte_tty_driver() call in the
module initialization function.

Signed-off-by: Quytelda Kahja <quytelda@tamalin.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>