Instead of silent acceptance of unsupported requested configuration
for PWM period and setting the boundary supported value, return
-ERANGE to a caller.

Duty period value equal to 0 or period is still accepted to allow
configuration by PWM sysfs interface, when it is set to 0 by default.

For reference this is a list of restrictions on period_ns == 1/freq:

  | PWM parent clock | parent clock divisor | max freq | min freq |
  +------------------+----------------------+----------+----------+
  |   HCLK == 13 MHz |      1 (min)         | 50.7 KHz | 198.3 Hz |
  |   HCLK == 13 MHz |     15 (max)         | 3.38 KHz | 13.22 Hz |
  |  RTC == 32.7 KHz |      1 (min)         |   128 Hz |   0.5 Hz |
  |  RTC == 32.7 KHz |     15 (max)         | 8.533 Hz | 0.033 Hz |

Note that PWM sysfs interface does not support setting of period more
than NSEC_PER_SEC / MAX_INT32 ~ 2 seconds, however this PWM controller
supports a period up to 30 seconds.
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>