drivers: net: Mark functions as static in debug.c

This patch marks the function il_clear_traffic_stats() in debug.c as
static because they are not used outside this file.

Thus, it also removes the following warnings in
wireless/iwlegacy/debug.c:

drivers/net/wireless/iwlegacy/debug.c:35:1: warning: no previous prototype for ‘il_clear_traffic_stats’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>