ath9k: fix some debugfs output

The right shift operation has higher precedence than the mask so we
left shift by "(i * 3)" and then immediately right shift by "(i * 3)"
then we mask.  It should be left shift, mask, and then right shift.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>