soc/amd/common/block/cpu/tsc_freq: move static variable inside function

Move the static mhz variable inside the only function that is accessing
it.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: Ief98c0a1c35fe1bbc4ff38dd175f12e0b3ddc515
Reviewed-on: https://review.coreboot.org/c/coreboot/+/74014
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>
Reviewed-by: Martin Roth <martin.roth@amd.corp-partner.google.com>

src/soc/amd/common/block/cpu/tsc/tsc_freq.c

@@ -7,8 +7,6 @@
 #include <console/console.h>
 #include <soc/msr.h>
 
-static unsigned long mhz;
-
 /* Use this default TSC frequency when it can not be correctly calculated.
    Higher numbers are safer as it will result in longer delays using TSC */
 #define TSC_DEFAULT_FREQ_MHZ	4000
@@ -17,6 +15,7 @@ unsigned long tsc_freq_mhz(void)
 {
 	union pstate_msr pstate_reg;
 	uint8_t high_state;
+	static unsigned long mhz;
 
 	if (mhz)
 		return mhz;