[uart] Remove ability to use frame formats other than 8n1

[uart] Remove ability to use frame formats other than 8n1

In the context of serial consoles, the use of any frame formats other
than the standard 8 data bits, no parity, and one stop bit is so rare
as to be nonexistent.

Remove the almost certainly unused support for custom frame formats.

Signed-off-by: Michael Brown <mcb30@ipxe.org>