mac80211: fix debugfs_sta print_mac() warning

mac80211: fix debugfs_sta print_mac() warning

When print_mac() was marked as __pure to avoid emitting a function
call in pr_debug() scenarios, a warning in this code surfaced since
it relies on the fact that the buffer is modified and doesn't use
the return value. This patch makes it use the return value instead.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reported-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>