pwm: Make pwmchip_[sg]et_drvdata() a wrapper around dev_set_drvdata()

pwm: Make pwmchip_[sg]et_drvdata() a wrapper around dev_set_drvdata()

Now that a pwm_chip has a dedicated struct device, pwmchip_set_drvdata()
and pwmchip_get_drvdata() can be made thin wrappers around
dev_set_drvdata() and dev_get_drvdata() respectively and the previously
needed pointer can be dropped from struct pwm_chip.

Link: https://lore.kernel.org/r/a5e05bd2d83421a26fdef6a87d69253c0f98becf.1710670958.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>