staging: wilc1000: remove spi speed control codes

This patch removes spi speed control codes. We are not using define SPEED to
specify speed of spi, it is not proper way of doing this. It will be
provided by the device tree.

The following functions and variable are removed.
MIN_SPEED, MAX_SPEED, SPEED
wilc_spi_set_max_speed
wilc_spi_max_bus_speed
wilc_spi_default_bus_speed
hif_set_max_bus_speed
hif_set_default_bus_speed

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