m68knommu: Fix support for console port other than ttyS0 on mcf.c

Due to a case of backwards logic, mfc.c always makes the console port be
ttyS0 even when you ask for another port.

This patch fixes this issue.

Only when the requested port is NOT in the range 0 to MAXPORTS-1 do we
force it to be treated as if port 0 was requested.  Forcing the port to
0 when it is in fact in the range 0 to MAXPORTS is not helpful.

Tested with working console on ttyS2 on a 5271evb.

Signed-off-by: Len Sorensen <lsorense@csclub.uwaterloo.ca>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>