loader.efi: Only use SPCR if enabled.

SerialPort in the SPCR is zeroed when serial redirection is disabled,
rather than the SPCR being omitted from the ACPI tables ony many
systems. Check to see that SerialPort.Address is non-zero before using.

FreeBSD would fail to boot on systems that could have a serial port
redireciton, but don't have it enabled because the loader would create a
bogus hw.uart.console. While one could unset this value to boot, you
couldn't do that automatically very easily. Instead, don't even look
at the SPCR table if the SerialPort is zero'd.

PR: 292206
MFC After: 3 days
Sponsored by: Netflix
Co-authored-by: Warner Losh <imp@FreeBSD.org>
Closes: https://github.com/freebsd/freebsd-src/pull/1948