WiFi-HAL: Initialize "wifi_read_packet_filter" return type

In earlier implementation of "wifi_read_packet_filter", there may arise a
scenario where return "ret" remain uninitialized if "length" value passed
to this function is zero and APF "enable/disable" functionality is carried out
by the framework.

Make changes, so that return "ret" is initialized at the time of its
declaration and a check is added where "length" to be passed to this function
is non-zero.

Bug: 121109906
Test: Regression Test
Change-Id: Ibc8bd164f086f8578b98b83eab3b2884188a4138
CRs-Fixed: 2263409