ath6kl: fix compiler warning in ath6kl_init_hw_params()

Both Luis and John reported that they see a compiler warning:

drivers/net/wireless/ath/ath6kl/init.c: In function 'ath6kl_init_hw_params':
drivers/net/wireless/ath/ath6kl/init.c:1377:26: warning: ‘hw’
may be used uninitialized in this function

Oddly enough I have never seen it. But AFAICT the code is correct and
hw is not used uninitalized so add uninitialized_var() to inform that to
the compiler.

Reported-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Reported-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>