staging: wilc1000: rename Session_index of struct pending_acks_info

This patch rename Session_index of struct pending_acks_info to session_index
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>