if_dwc: fix reinitialization with changed MAC

It was observed that on RockPro64 hardware, the dwc interface is unable
to receive packets after being assigned a new MAC address. The fix is
simply to call mii_mediachg() before touching any device registers in
dwc_init_locked(). This is consistent with what the OpenBSD driver does.

PR: 263820
MFC after: 1 week

(cherry picked from commit 6501fcdc0a97faa3c59f6ece314bf7754303db6f)