uart: Don't check SPCR tables if !late_console

On x86 systems, the debug.late_console tunable makes it possible to set
up the console before we call pmap_bootstrap. (The tunable is turned
on by default; setting late_console=0 results in consoles being probed
early.)

Unfortunately this is not compatible with using the ACPI SPCR table to
find the console, since consulting ACPI tables requires mapping memory
addresses. As such, we skip the call to uart_cpu_acpi_spcr from
uart_cpu_x86 in the !late_console case.

Reviewed by: imp
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D35794