ch32h417: make usbss_phy_cfg return void (unused read-back)

PVS-Studio (V530) flagged the discarded return value of usbss_phy_cfg at every
call site. The PHY back-door read-back is only a strobe; the value is never used
(matching the WCH EVT demo), so return void. PVS found no other issues in either
CH32H417 dcd.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QWhrtHjbSVMKcY68vxBwBm