x86 MTRR: Drop unused return value

It was never well-defined what value this function should return.

Change-Id: If84aff86e0b556591d7ad557842910a2dfcd3b46
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/6166
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

src/include/cpu/x86/mtrr.h

@@ -66,7 +66,7 @@ void enable_fixed_mtrr(void);
 void x86_setup_fixed_mtrrs(void);
 /* Set up fixed MTRRs but do not enable them. */
 void x86_setup_fixed_mtrrs_no_enable(void);
-int x86_mtrr_check(void);
+void x86_mtrr_check(void);
 #endif
 
 #if !defined(__ASSEMBLER__) && defined(__PRE_RAM__) && !defined(__ROMCC__)