cpufreq: unicore32: fix typo issue for 'clk'

cpufreq: unicore32: fix typo issue for 'clk'

Need use 'clk' instead of 'mclk', which is the original removed local
variable.

The related original commit:

  "652ed95 cpufreq: introduce cpufreq_generic_get() routine"

The related error with allmodconfig for unicore32:

    CC      drivers/cpufreq/unicore2-cpufreq.o
  drivers/cpufreq/unicore2-cpufreq.c: In function ‘ucv2_target’:
  drivers/cpufreq/unicore2-cpufreq.c:48: error: ‘struct cpufreq_policy’ has no member named ‘mclk’
  make[2]: *** [drivers/cpufreq/unicore2-cpufreq.o] Error 1
  make[1]: *** [drivers/cpufreq] Error 2
  make: *** [drivers] Error 2

Fixes: 652ed95d5fa6 (cpufreq: introduce cpufreq_generic_get() routine)
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: 3.14+ <stable@vger.kernel.org> # 3.14+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>