tty: max3100: use module_spi_driver

the driver's module init and exit functions can be replaced
with module_spi_driver as they do
only spi_register_driver and spi_unregister_driver in module's init and exit
paths.

Signed-off-by: Devendra Naga <develkernel412222@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>