staging: wilc1000: rename pMacHeader of function get_if_handler

This patch renames pMacHeader of function get_if_handler to mac_header
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>