staging: wilc1000: remove function pointer spi_max_speed

This patch removes function pointer spi_max_speed of wilc_spi_t and just call
the function linux_spi_set_max_speed directly.

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