term/ns8250-spcr: Return if redirection is disabled

term/ns8250-spcr: Return if redirection is disabled

The Microsoft spec for SPCR says "The base address of the Serial Port
register set described using the ACPI Generic Address Structure, or
0 if console redirection is disabled". So, return early if redirection
is disabled (base address = 0). If this check is not done we may get
invalid ports on machines with redirection disabled and boot may hang
when reading the grub.cfg file.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Leo Sandoval <lsandova@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>