cpufreq: sc520_freq: add 'fallthrough' to one case

Quieten an implicit-fallthrough warning in sc520_freq.c:

../drivers/cpufreq/sc520_freq.c: In function 'sc520_freq_get_cpu_frequency':
../include/linux/printk.h:343:2: warning: this statement may fall through [-Wimplicit-fallthrough=]
  printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
../drivers/cpufreq/sc520_freq.c:43:3: note: in expansion of macro 'pr_err'
   pr_err("error: cpuctl register has unexpected value %02x\n",
../drivers/cpufreq/sc520_freq.c:45:2: note: here
  case 0x01:

Fixes: bf6fc9fd ("[CPUFREQ] AMD Elan SC520 cpufreq driver.")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>