staging: wilc1000: remove spi_max_speed of wilc_wlan_io_func_t

This patch removes spi_max_speed of wilc_wlan_io_func_t which is not used
anymore and removes union u and struct spi, which does not have members in it.

Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>