cpufreq: governor: Ensure sufficiently large sampling intervals

After commit aa7519af (cpufreq: Use transition_delay_us for legacy
governors as well) the sampling_rate field of struct dbs_data may be
less than the tick period which causes dbs_update() to produce
incorrect results, so make the code ensure that the value of that
field will always be sufficiently large.

Fixes: aa7519af (cpufreq: Use transition_delay_us for legacy governors as well)
Reported-by: Andy Tang <andy.tang@nxp.com>
Reported-by: Doug Smythies <dsmythies@telus.net>
Tested-by: Andy Tang <andy.tang@nxp.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>