pwm: raspberrypi-poe: Drop assignment to struct pwmchip::base

Since commit f9a8ee8c ("pwm: Always allocate PWM chip base ID
dynamically") there is no effect any more for assigning this variable.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>