ath9k: Fix compile warnings when DEBUGFS is disabled.

This fixes two compile warnings, and removes a useless
cast when assigning the 'sc' variable.

Reported-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>