staging: wilc1000: rename u32TailLen of struct beacon_attr

This patch renames u32TailLen of struct beacon_attr to tail_len 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>