ath9k: fix SC_OP_INVALID test in ath9k_tx99_init()

SC_OP_INVALID is zero so the test is always false.  We're supposed to be
testing the lowest bit instead.

Fixes: 89f927af ('ath9k: add TX99 support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>