staging: wilc1000: rename u8keyidx of struct host_if_wpa_attr

This patch renames u8keyidx of struct host_if_wpa_attr to index to
avoid CamelCase naming convention.

Signed-off-by: Leo Kim <leo.kim@atmel.com>
Signed-off-by: Tony Cho <tony.cho@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>