i386: fix PGE mask

cr4 is a 32-bit register, so casting the mask to an unsigned char is wrong,
as it clears more than the PGE bit.

Signed-off-by: Brian Gerst <bgerst@didntduck.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>