uart/pci: recover ADL AMT device after FIFO size probing

When the Alder Lake Serial-over-LAN device is put into loopback mode and
repeated writes are performed to the data register it results in the device
ending up in a non-functional state afterwards.

Recovering the device to a working state requires re-writing the LCR
register with it's current value (no effective change). This should be
harmless on all other devices.

Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D56107
Reviewed by: imp