mt7915: downgrade WA firmware to 20230315163130

This version I sourced from EAP615 WALL GPL source dump fixes the issue
of random crashes, in particular these issues:

- openwrt/mt76#776
- openwrt/mt76#690

Firmware downloaded from:

https://static.tp-link.com/upload/gpl-code/2023/202305/20230515/eap615-wall_gpl_code.tar.gz