pwm: tiehrpwm: Fix runtime PM imbalance at unbind

Remove unbalanced RPM put at driver unbind which resulted in a negative
usage count.

Fixes: 19891b20 ("pwm: pwm-tiehrpwm: PWM driver support for EHRPWM")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>