net/wireless/ti/wlcore/spi: Use module_spi_driver to register driver

Removing some boilerplate by using module_spi_driver instead of calling
register and unregister in the otherwise empty init/exit functions

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Luciano Coelho <coelho@ti.com>