x86/Kconfig: Correct reference to MWINCHIP3D

Commit in Fixes intended to exclude the Winchip series and referred to
CONFIG_WINCHIP3D, but the config symbol is called CONFIG_MWINCHIP3D.

Hence, scripts/checkkconfigsymbols.py warns:

WINCHIP3D
Referencing files: arch/x86/Kconfig

Correct the reference to the intended config symbol.

Fixes: 69b8d3fc

 ("x86/Kconfig: Exclude i586-class CPUs lacking PAE support from the HIGHMEM64G Kconfig group")
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20210803113531.30720-4-lukas.bulwahn@gmail.com