ACPI SPCR: handle BaudRate=0

From https://github.com/tianocore/edk2-platforms/commit/7d8dc6544c93a5f5a03c83316489ba8700946e9f

"The mcbin (and likely others) have a nonstandard uart clock. This means
that the earlycon programming will incorrectly set the baud rate if it is
specified. The way around this is to tell the kernel to continue using the
preprogrammed baud rate. This is done by setting the baud to 0."

Our drivers (uart_dev_ns8250) do respect zero, but SPCR would error. Let's
not error.

Submitted by: Greg V <greg@unrelenting.technology>
Reviewed by: mw, imp, bcran
Differential Revision: https://reviews.freebsd.org/D19914