iwlwifi: fix compilation warning when CONFIG_IWLWIFI_DEBUG is not set.

When CONFIG_IWLWIFI_DEBUG is not set and CONFIG_IWLWIFI is set,
we get this compilation warning:
/wireless-next-2.6/drivers/net/wireless/iwlwifi/iwl-agn-rs.c: In
function 'rs_free_sta':
/wireless-next-2.6/drivers/net/wireless/iwlwifi/iwl-agn-rs.c:2425:
warning: unused variable 'priv'

This patch fixes it by adding __maybe_unused attribute.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>