iwlwifi: mvm: fix an out-of-bound access

iwlwifi: mvm: fix an out-of-bound access

The index for the elements of the ACPI object we dereference
was static. This means that if we called the function twice
we wouldn't start from 3 again, but rather from the latest
index we reached in the previous call.
This was dutifully reported by KASAN.

Fix this.

Cc: stable@vger.kernel.org
Fixes: aa18dab5f701 ("iwlwifi: mvm: add support for EWRD (Dynamic SAR) ACPI table")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>