ath9k_hw: fix uninitialized variable data

Currently, data variable in ar9003_hw_thermo_cal_apply() could be
uninitialized if ar9300_otp_read_word() will fail to read the value.
Initialize data variable with 0 to prevent an undefined behavior. This
will be enough to handle error case when ar9300_otp_read_word() fails.

Fixes: 80fe43f2

 ("ath9k_hw: Read and configure thermocal for AR9462")
Cc: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Cc: John W. Linville <linville@tuxdriver.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: stable@vger.kernel.org
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>