pwm: sifive: Always let the first pwm_apply_state succeed

Commit 2cfe9bbe added support for the
RGB and green PWM controlled LEDs on the HiFive Unmatched board
managed by the leds-pwm-multicolor and leds-pwm drivers respectively.
All three colours of the RGB LED and the green LED run from different
lines of the same PWM, but with the same period so this works fine when
the LED drivers are loaded one after the other.

Unfortunately it does expose a race in the PWM driver when both LED
drivers are loaded at roughly the same time. Here is an example:

  |          Thread A           |          Thread B           |
  |  led_pwm_mc_probe           |  led_pwm_probe              |
  |    devm_fwnode_pwm_get      |                             |
  |      pwm_sifive_request     |                             |
  |        ddata->user_count++  |                             |
  |                             |    devm_fwnode_pwm_get      |
  |                             |      pwm_sifive_request     |
  |                             |        ddata->user_count++  |
  |         ...                 |          ...                |
  |    pwm_state_apply          |    pwm_state_apply          |
  |      pwm_sifive_apply       |      pwm_sifive_apply       |

Now both calls to pwm_sifive_apply will see that ddata->approx_period,
initially 0, is different from the requested period and the clock needs
to be updated. But since ddata->user_count >= 2 both calls will fail
with -EBUSY, which will then cause both LED drivers to fail to probe.

Fix it by letting the first call to pwm_sifive_apply update the clock
even when ddata->user_count != 1.

Fixes: 9e37a53e ("pwm: sifive: Add a driver for SiFive SoC PWM")
Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>