MIPS: Fix FTLB detection for R6

MIPS: Fix FTLB detection for R6

R6 removed the Config4.MMUExtDef field, with the low 16 bits only
allowed to contain FTLB fields, and commit e87569cd6c57 ("MIPS:
cpu-probe: Fix VTLB/FTLB configuration for R6") updated the probing of
this field to assume an FTLB is always present for R6.

However the FTLB may still be absent. The presence of those fields is
actually specified by the MMU type in the Config.MT field, so use that
(the new cpu_has_ftlb) to determine whether the FTLB is actually
present.

Fixes: e87569cd6c57 ("MIPS: cpu-probe: Fix VTLB/FTLB configuration for R6")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11160/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>