ath10k: initialize fw_features var

If firmware did not have any feature flags set the
var would be left with values found on the stack
(i.e. garbage) yielding print string like this:

  (...) features \xffffffa6m:^R\xfffffffbԂ\xffffffc4^E

Fixes: b27bc5a4 ("ath10k: dump fw features during probing")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>