efi_loader: fix text output for Uart() DP nodes

The UEFI specification concerning Uart() device path nodes has been
clarified:

Parity and stop bits can either both use keywords or both use
numbers but numbers and keywords should not be mixed.

Let's go for keywords as this is what EDK II does. For illegal
values fall back to numbers.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>