staging: wilc1000: rename u8NumRates of struct add_sta_param

This patch renames u8NumRates of struct add_sta_param to rates_len
to avoid CamelCase naming convention.

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