pwm: ntxec: Drop useless 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.
When the patch resulting in f9a8ee8c

 was created, this driver didn't
exist yet, so this was missed.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>