ath9k_hw: avoid possible infinite loop in ar9003_get_pll_sqsum_dvc

ath9k_hw: avoid possible infinite loop in ar9003_get_pll_sqsum_dvc

"ath9k: Fix softlockup in AR9485" with commit id
64bc1239c790e051ff677e023435d770d2ffa174 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: [email protected]
Cc: Rolf Offermanns <[email protected]>
Cc: Sujith Manoharan <[email protected]>
Cc: Senthil Balasubramanian <[email protected]>
Signed-off-by: Mohammed Shafi Shajakhan <[email protected]>
Signed-off-by: John W. Linville <[email protected]>