ath9k_hw: avoid possible infinite loop in ar9003_get_pll_sqsum_dvc

"ath9k: Fix softlockup in AR9485" with commit id
64bc1239 fixed the reported
issue, yet its better to avoid the possible infinite loop
in ar9003_get_pll_sqsum_dvc by having a timeout as suggested
by ath9k maintainers.
http://www.spinics.net/lists/linux-wireless/msg92126.html

.
Based on my testing PLL's locking measurement is done in
~200us (2 iterations).

Cc: stable@vger.kernel.org
Cc: Rolf Offermanns <rolf.offermanns@gmx.net>
Cc: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Cc: Senthil Balasubramanian <senthilb@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>