staging: wilc1000: rename au8Interval of struct join_bss_param

This patch renames au8Interval of struct join_bss_param to interval
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>