cpufreq: Remove cpufreq_driver check in cpufreq_boost_supported()

Currently there are three calling paths for cpufreq_boost_supported() in
all as below, we can see the cpufreq_driver null check is needless since
it is already checked before.

<path1>
  cpufreq_enable_boost_support()
    |-> if (!cpufreq_driver)
    |-> cpufreq_boost_supported()

<path2>
  cpufreq_register_driver()
    |-> if (!driver_data ...
    |-> cpufreq_driver = driver_data
    |-> cpufreq_boost_supported()
    |-> remove_boost_sysfs_file()
          |-> cpufreq_boost_supported()

<path3>
  cpufreq_unregister_driver()
    |-> if (!cpufreq_driver ...
    |-> remove_boost_sysfs_file()
          |-> cpufreq_boost_supported()

Signed-off-by: Yue Hu <huyue2@yulong.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>