pwm: sifive: Shut down hardware only after pwmchip_remove() completed

The PWMs are expected to be functional until pwmchip_remove() is called.
So disable the clks only afterwards.

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